Connect a Microsoft 365 Mailbox to WarmySender

Overview

This guide walks you through every way to connect a Microsoft 365 mailbox (or an Outlook.com personal account) to WarmySender. There are three methods, and we recommend the first one for almost everyone:

  1. Sign in with Microsoft (recommended) — one click, no admin setup, no password ever leaves Microsoft.
  2. App Password — paste an email and an app password into our form. Only works if your Microsoft admin has Authenticated SMTP turned on.
  3. Custom IMAP/SMTP — for non-standard Exchange setups or self-hosted Microsoft environments.

The rest of this page walks you through each method, then shows you how to connect many Microsoft mailboxes back-to-back (a common gotcha where the browser silently signs you in as the wrong account), then covers troubleshooting for the most common errors, the permissions we ask for, and our privacy and security posture.

WarmySender is a 4-pillar outreach platform — Cold Emailing, Email Warmup, LinkedIn Outreach, and Multichannel sequences. Microsoft 365 mailboxes can be used for cold emailing and for email warmup; LinkedIn outreach is handled separately on your LinkedIn account.

Method 1: Sign in with Microsoft (recommended)

One click. No admin setup. No password leaves Microsoft. Works for any Microsoft 365 work or school account, any Outlook.com personal account, and most Exchange Online setups.

Steps

  1. Open Mailboxes in WarmySender and click Add Mailbox.
  2. Choose Sign in with Microsoft.
  3. Type the email address of the mailbox you want to connect into the Which email do you want to connect? field. This tells Microsoft to show you the account picker so the correct account is highlighted.
  4. Click Continue. Microsoft will open in a new tab.
  5. Pick the account from Microsoft's account picker (or sign in to a different account if it's not listed).
  6. Review the permissions we ask for (see What permissions WarmySender asks for below) and click Accept.
  7. You'll be returned to WarmySender. The new mailbox appears in your list with a green Connected badge.

Why we recommend this method

Connecting many Microsoft mailboxes back-to-back

If you're connecting more than one Microsoft mailbox in a single sitting, read this section before you start. There's a subtle Microsoft browser behavior that can silently connect the wrong account, and we have three easy ways to prevent it.

Why this happens

When you sign in to a Microsoft account in your browser, Microsoft remembers that account. The next time any website asks you to sign in with Microsoft, Microsoft may quietly re-use that same account without showing you the account picker. If you've just finished connecting your first mailbox to WarmySender and immediately click Add Mailbox again, Microsoft may silently re-sign you in as the first account — and you'll end up with the same mailbox connected twice, with no error message.

Three ways to avoid the wrong-account problem

  1. Use the "Which email do you want to connect?" field. Type the email address of the next mailbox you want to add. We pass that to Microsoft as a hint, and Microsoft will always show you the account picker (and pre-select that account). This is the easiest path — works for connecting two mailboxes or fifty.
  2. Click "Sign out of Microsoft first" in the connect dialog. Before you start a new connect, click this one-click link in the WarmySender connect dialog. It signs you out of Microsoft in your current browser, so the next connect always starts from a clean slate and Microsoft shows you the picker.
  3. Open each connect in a new private/incognito window. An incognito window has no Microsoft session, so the picker always appears. Slowest option, but works in browsers without the in-app "Sign out of Microsoft" link.

What "connected the wrong account" looks like

There's no error message — that's what makes this confusing. You'll see one of these symptoms:

How to fix it if it already happened

  1. Open Mailboxes and click Disconnect on the wrong row (or the duplicate row).
  2. Click Sign out of Microsoft first in the connect dialog (or open a new incognito window).
  3. Click Add Mailbox › Sign in with Microsoft again, and this time type the correct email into the Which email do you want to connect? field.
  4. Microsoft will now show you the account picker. Pick the right account.

Method 2: App Password (Microsoft 365)

Use App Password if you've already generated one or if you'd prefer not to grant the Microsoft sign-in permissions. This method works only when your Microsoft 365 admin has Authenticated SMTP turned on for your account. If you're not sure whether it's on, try Method 1 first — it bypasses this requirement entirely.

Steps

  1. Make sure your Microsoft 365 admin has Authenticated SMTP enabled for your mailbox. If you're the admin yourself, the steps are below in How an admin enables Authenticated SMTP.
  2. Make sure multi-factor authentication is on for the account. (You can't generate an app password unless MFA is on.)
  3. Sign in to mysignins.microsoft.com with the mailbox account.
  4. Click Security info › Add sign-in method › App password.
  5. Give the app password a name (something like "WarmySender") and click Next. Microsoft shows you a 16-character password — copy it.
  6. In WarmySender, open Mailboxes › Add Mailbox › App Password (Microsoft).
  7. Paste your email address and the app password you just copied.
  8. Leave the server and port fields as they are — they're pre-filled with the correct values for Microsoft 365. Don't change them unless you know your tenant uses non-standard settings.
  9. Click Connect. The mailbox should connect within a few seconds.

How an admin enables Authenticated SMTP

If you're the Microsoft 365 admin (or you can forward these steps to your admin), here's the per-user fix that takes 30 seconds:

  1. Sign in to the Microsoft 365 Admin Center with a Global Administrator or Exchange Administrator account.
  2. Open Users › Active Users.
  3. Click the user whose mailbox you're trying to connect.
  4. Open the Mail tab.
  5. Click Manage email apps.
  6. Tick the box next to Authenticated SMTP and click Save changes.
  7. Wait about a minute for Microsoft to apply the change, then try the App Password connect in WarmySender again.

For the full admin runbook (including the tenant-wide alternative, why Microsoft disables this by default, and what to do if your admin refuses to enable it), see our companion guide: SMTP disabled by Microsoft admin — full fix guide.

Method 3: Custom IMAP/SMTP

This is only for non-standard Microsoft Exchange setups — on-premises Exchange behind a custom relay, a hybrid Exchange + Microsoft 365 deployment with a non-default routing, or a Microsoft-backed mailbox served through a third-party provider with custom server settings.

If you don't know whether you need this, you almost certainly don't — use Method 1 (Sign in with Microsoft) instead. If you do know you need it, see our general guide: Connecting your mailbox (custom IMAP/SMTP) for the full field reference.

Troubleshooting

"Authentication unsuccessful" when using App Password

This error almost always means Microsoft has Authenticated SMTP turned off — either at the tenant level (for your whole organization) or just for your specific account. Microsoft's error message says "credentials were incorrect," which is misleading: the password is fine, but the door is closed.

The fastest fix: switch to Method 1 (Sign in with Microsoft). One click, no admin action needed. You'll see a Connect with Microsoft instead button on the error message — clicking it takes you directly to the Microsoft sign-in flow.

If you must use App Password: ask your IT admin to enable Authenticated SMTP for your account using the steps in How an admin enables Authenticated SMTP above. Full admin runbook: SMTP disabled by Microsoft admin.

"The wrong Microsoft account got connected"

You were already signed in to Microsoft as a different account in your browser when you started the connect, and Microsoft silently re-used that session. See Connecting many Microsoft mailboxes back-to-back above for the three ways to prevent this. To recover: disconnect the wrong mailbox, then re-add it using the Which email do you want to connect? field, or after clicking Sign out of Microsoft first in the connect dialog, or in a new incognito window.

"I don't see the new mailbox in my list after Microsoft sign-in"

Same root cause as above — Microsoft silently re-used your existing browser session and signed you back in as a mailbox you already have connected. The platform recognized it as a duplicate and didn't create a new row. Same fix: disconnect that account, sign out of Microsoft (or open incognito), and try again with the correct email in the connect field.

"My organization blocks third-party app sign-ins"

Your Microsoft 365 organization restricts which third-party apps employees can sign into. The fastest path is the in-app admin-approval flow described in the next section: What if Microsoft shows "Need admin approval"?. Once admin approval is recorded once for your organization, anyone in your organization can connect their mailbox normally — they won't see the approval prompt again. If you'd like a one-page brief to forward to your admin, email [email protected] and we'll send one.

"My Microsoft mailbox keeps showing 'Reconnect needed'"

If you connected via App Password and Microsoft revoked the password (commonly after a security event like an MFA reset or admin policy change), you'll need to generate a fresh app password and re-connect. The easier long-term fix is to switch to Method 1 (Sign in with Microsoft), which refreshes its session automatically and won't break the next time MFA settings change.

What if Microsoft shows "Need admin approval"?

When you try to sign in, Microsoft may say "Need admin approval". This happens when your organization restricts which apps employees can sign into — a common setting for business and enterprise Microsoft 365 plans. It's not an error with your account or with WarmySender; your organization just needs to approve the app once.

There are two ways to fix this:

Option 1 (recommended) — Use the in-app admin approval button

  1. Go to Mailboxes in WarmySender.
  2. Click Add Mailbox and expand "Having trouble? My organization requires admin approval".
  3. Click "Grant org-wide admin consent".
  4. Sign in as a Microsoft admin (Global Administrator, Application Administrator, or similar).
  5. Approve WarmySender for your organization.
  6. You'll see a confirmation message. Close the window and connect your mailbox normally.

Option 2 — Have your IT admin approve from Microsoft Entra

  1. Ask your IT admin to sign in at the Microsoft Entra admin center.
  2. Go to Enterprise Applications and search for "WarmySender".
  3. Click the app, open Permissions, and click "Grant admin consent for [your organization]".
  4. Anyone in your organization can then connect their mailbox normally.

After admin approval is done once, every user in your organization can connect their own mailbox without seeing the approval prompt. You only have to do this once per organization, not once per user.

"You signed in as X, but the mailbox you clicked is Y"

You may see a message like this if an admin signed in to grant approval (which is correct) but the original mailbox row in your list belongs to a different user. Your organization's approval has been recorded — that part worked. To finish connecting the original mailbox:

  1. Go back to Mailboxes.
  2. Click Reconnect on the original mailbox row.
  3. Sign in as that specific user (not the admin).

Because the admin already approved WarmySender for the whole organization in the previous step, this sign-in will go through without any approval prompt.

What permissions WarmySender asks for

When you use Sign in with Microsoft (Method 1), Microsoft will show you a consent screen listing the permissions we ask for. In plain language, here's what they are and why we need them:

Read your name, email address, and profile picture
So we can show your mailbox in your account list with your name and avatar, and so you can pick the right mailbox when you have several connected.
Send mail on your behalf
This is the core permission. Without it, WarmySender can't send your cold email campaigns or warmup emails.
Read your mail and folders
So we can detect replies (and route them to your inbox in WarmySender), recognize warmup messages and move them out of spam, and track bounces. We only look at messages relevant to your outreach — we never read or store the contents of your personal email.
Stay signed in (offline access)
So your mailbox stays connected without you needing to sign in every day. Microsoft refreshes the session automatically; you can revoke access at any time.

You can review and revoke these permissions at any time from your Microsoft account: myapps.microsoft.com › Apps that have access to my account.

Privacy and security

We never store your Microsoft password. When you use Sign in with Microsoft, your password goes straight from your browser to Microsoft and never touches our servers — we only receive an encrypted session token that lets us send and read mail on your behalf. App passwords are stored encrypted at rest. You can disconnect any mailbox at any time from the Mailboxes page in WarmySender (the disconnect is instant and revokes our access immediately). We only read messages we need to for the product to work — replies to your campaigns, warmup mail, and bounce notifications — never your personal email. Full details are in our Privacy Policy and Security page.

Frequently asked questions

Which connection method should I use?

Sign in with Microsoft (Method 1) for almost everyone. It's one click, no admin setup, no password to manage, and it survives password changes. Use App Password (Method 2) only if you've already set one up or you have a specific reason not to use Microsoft sign-in. Use Custom IMAP/SMTP (Method 3) only for non-standard Exchange setups.

Do I need an admin to set this up?

For Method 1 (Sign in with Microsoft) — usually no. Personal Outlook.com accounts just work. Most Microsoft 365 work and school accounts allow third-party sign-ins by default. For Method 2 (App Password) — yes, your admin needs to enable Authenticated SMTP for your account first. That's another reason we recommend Method 1.

Can I connect lots of Microsoft mailboxes at once?

Yes — you can connect as many as your plan allows (Pro 50, Business 200, Enterprise 400, Ultimate 800 per workspace). When connecting many back-to-back, read the Connecting many Microsoft mailboxes back-to-back section above to avoid the silent wrong-account problem.

Does this work for personal Outlook.com accounts?

Yes. Personal Outlook.com, Live.com, and Hotmail.com accounts all work with Method 1 (Sign in with Microsoft). The flow is identical — sign in, pick the account, accept the permissions, done.

How do I disconnect a Microsoft mailbox?

Open Mailboxes in WarmySender, find the row, and click Disconnect. The connection is revoked immediately. To also revoke our access at Microsoft's end, visit myapps.microsoft.com and remove WarmySender from the apps list.

Can I connect a mailbox from one organization and another from a different organization?

Yes. Each connect is independent. Sign in to Microsoft as the first account, accept, return to WarmySender. Then for the second account, use the Which email do you want to connect? field or sign out of Microsoft first, and sign in as the second account from a different organization.

Can I bulk-import many Microsoft mailboxes at once?

Bulk import (paste a CSV) supports password and app-password based connections. Microsoft sign-in (Method 1) is one mailbox per click because Microsoft's sign-in flow is per-account by design. For mixed batches, see our Bulk import mailboxes guide.

Is the mailbox eligible for warmup immediately after connecting?

Yes. As soon as the green Connected badge appears, you can turn warmup on for the mailbox from the Mailboxes page. New mailboxes start warmup at a low daily volume and ramp up over 4-6 weeks to protect your sender reputation. See Email warmup for more.

Still stuck after trying the steps above? Email [email protected] with the mailbox address and a screenshot of the error you saw — we'll help you connect it.