How to connect a Microsoft 365 alias to WarmySender

Overview

If you run Microsoft 365 or Exchange Online, you can point several alias addresses at WarmySender while they all sign in through one licensed mailbox. Each alias then behaves like its own sender: its own From address, its own warmup, its own daily limit, and its own campaigns — without a separate Microsoft licence for every outreach domain.

This is the Microsoft equivalent of connecting a Google Workspace alias. The idea is identical; only the server names and the two tenant settings differ.

What counts as an alias here

A Microsoft 365 mailbox has one primary address you sign in with, and can carry additional addresses that all deliver into that same inbox. Microsoft calls those additional addresses aliases (or proxy addresses), and they can sit on completely different domains once those domains are added to your organisation.

So a single licensed mailbox — say [email protected] — might also answer to [email protected], [email protected] and [email protected]. In WarmySender each of those becomes its own mailbox entry, and all of them sign in as [email protected].

Use Custom IMAP/SMTP, not the one-click Microsoft sign-in

This is the step that catches most people. Sign in with Microsoft connects exactly one address: the mailbox that signed in. There is no place in that flow to add an alias, because the sign-in itself defines the sending address. If you have already connected the licensed mailbox that way and are looking for a way to add its aliases, you will not find one — that is expected, and it is not a limitation of alias support.

Choose Custom IMAP/SMTP instead. That form has a separate Email address field and Username field, and for an alias they are deliberately different values.

The settings to enter

The mental model is worth holding on to: Username is who signs in, Email address is who the message comes from. They are separate fields on purpose, and for an alias they are supposed to hold different values.

Use port 587 — not 465

Microsoft accepts outgoing mail from applications on port 587 (or 25). It does not serve port 465 for this kind of connection, so a mailbox set to 465 will simply sit there and time out rather than return a clear error. If a Microsoft mailbox looks stuck with connection problems and nothing about the password seems wrong, check the outgoing port first. WarmySender now corrects this combination for you when it sees it, but it is worth knowing why.

Port 465 is perfectly normal on many other providers — this is specific to Microsoft.

Two settings your administrator may need to switch on

Microsoft turns both of these off by default for organisations created after January 2020, so a brand-new tenant almost always needs them enabled once:

1. Authenticated SMTP — this is what lets an application send through the licensed mailbox at all.
2. IMAP — this is what lets WarmySender read incoming mail, which is how warmup confirms your messages arrived and how replies are detected.

Both live in the same place in the Microsoft 365 admin centre: Users → Active users → select the licensed mailbox → Mail → Manage email apps. Tick Authenticated SMTP and IMAP, then save. You only need to do this for the licensed mailbox that signs in — not for each alias, which has no sign-in of its own.

If reading incoming mail stays switched off, the mailbox can still send, but warmup cannot confirm delivery and replies will not be picked up. See why warmup needs incoming access.

Connect one alias first and prove it

Microsoft — not WarmySender — decides whether it will accept an alias in the From line of a message submitted by an application. That behaviour depends on how your organisation is configured, so the only reliable way to know is to try it once.

Connect a single alias exactly as described above, run Test connection, save it, then send one real test message and confirm it arrives showing the alias as the sender. Once that works, add the rest with confidence. If Microsoft refuses to send as the alias, your administrator will need to grant that address permission to send from the licensed mailbox.

What each alias gets in WarmySender

Every alias you connect appears as its own mailbox, with:

• its own From address and sender name;
• its own warmup ramp and warmup schedule;
• its own daily sending limit;
• its own campaigns and its own reporting.

Aliases count towards your plan's mailbox allowance the same way any other mailbox does. See mailbox limits for what your plan includes, and how warmup ramps up for what to expect in the first days.

The one thing to plan around: aliases share the mailbox's ceiling

This is the most important caveat, and it is easy to miss. Microsoft applies its sending limits to the licensed mailbox, not to each alias. Ten aliases on one licensed mailbox are still one mailbox as far as Microsoft is concerned, and they draw on one shared allowance.

WarmySender paces all of them together for that reason, so the aliases on one licensed mailbox cannot collectively outrun what that mailbox is actually allowed to send. That keeps the account safe, but it does mean that stacking more aliases onto one licensed mailbox does not increase how much you can send. If you need more total volume, add more licensed mailboxes rather than more aliases on the same one.

There is a second, smaller effect worth knowing: because every alias delivers into the same physical inbox, warmup has to pick your messages out of one shared stream. It works, but confirmation is a little less precise than when each sender has an inbox of its own. Keeping the number of aliases per licensed mailbox sensible helps here too.

Connecting several aliases at once

Add each alias as its own mailbox, changing only the Email address field each time — the Username and password stay the same for every one of them. If you are adding a lot of them, bulk import accepts a separate username column alongside email, which is exactly the alias shape: put the alias in email and the licensed mailbox in username.

Troubleshooting

It says the sign-in was rejected

The most common cause is the alias address sitting in the Username field. Microsoft authenticates the licensed mailbox, not the alias, so the Username must be the licensed mailbox even though the Email address above it is the alias. The second most common cause is Authenticated SMTP not being switched on yet for that mailbox.

It hangs and then says the server did not respond

Check the outgoing port is 587. Microsoft does not answer on 465 for this kind of connection, so a mailbox set to 465 waits and then gives up.

It sends fine but warmup never confirms anything arrived

IMAP is switched off for the licensed mailbox. Turn it on under Manage email apps as described above; nothing needs re-typing in WarmySender once it is on.

Can I add aliases to a mailbox I already connected with Sign in with Microsoft?

Not to that entry, no — that connection is bound to the address that signed in. Add each alias separately using Custom IMAP/SMTP as described above. You can keep the one-click connection for the licensed mailbox itself and add the aliases alongside it.

Do I need a Microsoft licence for every alias?

Aliases belong to the licensed mailbox they sit on, so you are not creating new mailboxes on Microsoft's side. What you do need is enough mailbox allowance in your WarmySender plan, since each alias is a separate sender here — see mailbox limits.

Related guides

• Full documentation — every WarmySender guide in one place
• Connect a Microsoft 365 mailbox — the three ways to connect a standard (non-alias) Microsoft mailbox
• Connect a Gmail or Google Workspace alias — the same setup on Google
• Bulk import mailboxes (CSV + paste) — connect many alias mailboxes at once
• Why warmup needs incoming access — what breaks when reading incoming mail is switched off
• Mailbox limits — how many mailboxes your plan includes
• How warmup ramps up — what to expect in your first hour, first day, first week
• Support — email [email protected] if you're stuck