Why is my email blocked by Microsoft or Outlook?
If Microsoft or Outlook is rejecting your email, the cause is almost always the reputation of the IP address your mail is SENT FROM — not your warmup. Email warmup does not block an IP, and it cannot unblock one either. Outlook.com and Office 365 decide whether to accept a message largely on what they know about the sending IP and the sending domain, so a hard rejection from Microsoft is a decision about the IP your mail left from, made before warmup ever enters the picture. This is one of the most common questions we get, and it usually is not a sign that you did anything wrong.
Why warmup is not the cause:
Warmup sends a small, gradually increasing number of messages to a network of real inboxes that genuinely open, reply to, and engage with them. It never sends in bulk to Microsoft on your behalf, and it cannot get an IP blocked. In fact it works in the opposite direction — over time it builds positive signals that improve how mailbox providers see your domain. If Microsoft has flagged the IP you send from, that flag came from the broader sending history on that IP, and warmup is simply delivering through it like every other message you send.
The usual real cause is a shared sending IP:
Many domains on web or cPanel shared hosting — for example Namecheap, Hostinger, or GoDaddy plans — send their outbound mail through an IP address shared with dozens or hundreds of other unrelated websites. You do not control that IP, and neither do the other tenants on it.
Microsoft is especially strict about shared hosting IP ranges, because the reputation of that IP is the SUM of everyone sending from it. If another website on the same IP sends spam or collects complaints, Microsoft can throttle or block the entire IP — and your perfectly clean mail gets caught in the same net, no matter how good your content or list is. The problem is not your sending; it is the IP your mail leaves from.
How to confirm it:
Read the exact bounce or error message Microsoft returns. A shared-IP block usually names the specific IP that was refused and links to a page explaining the reason and how to request a review. The exact wording varies, but a rejection of this kind generally reads like this generic example: "Your message could not be delivered. The sending IP address has been refused by the recipient's mail service because of its reputation. Please visit the listed support page to request a review." The pattern is consistent — it points at an IP, not at your warmup, your content, or your subject line. If that IP belongs to your hosting provider's shared server rather than an address you personally manage, you have your answer.
How to fix it, in priority order:
- Send from a mailbox that has its own trusted reputation. This is the single most reliable fix. Connecting a Microsoft 365 or Google Workspace mailbox directly means your mail leaves through Microsoft's or Google's own carefully managed sending infrastructure instead of a shared web-hosting IP. See "Connect a Microsoft 365 Mailbox to WarmySender" for the steps.
- If you want to keep your own domain's mail server, ask your hosting provider whether you are on a shared sending IP and whether a dedicated IP is available. A dedicated IP, or a reputable managed sending service, gives you reputation you actually control.
- Make sure SPF, DKIM, and DMARC are correctly configured for your domain. These are necessary but not sufficient on their own — passing authentication does not override a blocked IP. See "Email Deliverability (SPF, DKIM, DMARC)" for setup help.
- If the bounce includes a delisting or mitigation link, follow it and submit the request. On a shared IP you do not fully control the outcome, because the IP's reputation also depends on the other senders on it — which is why moving to a mailbox with its own reputation is the durable fix, not repeated removal requests.
What warmup can and cannot do:
Warmup CAN:
- Build your domain and content reputation through real engagement.
- Keep your mailbox active and looking human, which providers reward over time.
- Improve inbox placement with providers that weigh sender engagement.
Warmup CANNOT:
- Change the reputation of an IP you share with other senders — you do not control it, so warmup cannot fix it.
- Force a provider that has blocked your sending IP to start accepting your mail.
A quick way to remember it: warmup builds the reputation of YOU — your domain and your sending behavior — but cannot rebuild the reputation of an IP shared with strangers. Pairing warmup with a mailbox that has its own trusted IP gives you both halves. To watch your domain's standing improve as warmup builds it, see "Domain Reputation Monitoring".
Still stuck? Contact us at [email protected] and include the exact bounce or error text you received — that message is the single most useful clue for pinpointing the cause. For instant answers, you can also open warmysender.com/documentation, click "Copy Prompt to Clipboard," and paste it into an AI assistant like Claude or ChatGPT along with your question.