“You weren't recognized as a valid sender”: Microsoft blocked my email
Microsoft 365 has paused your mailbox from sending, because it sent more than Microsoft allows or sent something that looked like spam. You can still receive email. The most common cause is a hacked account, so check that first. Then whoever manages your Microsoft 365 can unblock you, and Microsoft says sending usually works again within an hour.
Check your own email first. Free, about 90 seconds. You get a grade and a plain-English list of what's working and what isn't.
The message you’re seeing
Your emails bounce back with something like this:
Your message couldn’t be delivered because you weren’t recognized as a valid sender. The most common reason for this is that your email address is suspected of sending spam and it’s no longer allowed to send email. Contact your email admin for assistance. Remote Server returned ’550 5.1.8 Access denied, bad outbound sender.’
Why this happens
Microsoft caps how much one mailbox can send: up to 10,000 people a day and 30 messages a minute. Go over, or send something its filters think is spam, and the mailbox is paused. Microsoft treats this as a sign the account may have been taken over, which is why it asks for a security check before unblocking.
What to do right now
- Stop trying to send. Retrying won’t get through.
- Look in your Sent folder for emails you didn’t write. Also check for rules that forward or move your email that you didn’t set up.
- Tell whoever manages your Microsoft 365. That might be your IT person, your web person, or you. Forward them this page. The steps for them are below.
- Once you’re unblocked, change your password and turn on two-step sign-in, even if nothing looked wrong.
- For anything urgent in the meantime, pick up the phone.
Did this happen after you emailed a big list from your mailbox? Read what to do after a newsletter from your regular email so it doesn’t happen again.
What this guide can’t do for you
It covers the common causes. It can’t tell you which of your tools is the one that’s failing, or give you the exact settings for your business and the company that manages your domain. That’s what the Email Fix does. We check every tool that sends email for you and give you steps written for your setup. If we can't find what's wrong with your email settings, you get your $299 back. The guarantee covers your email settings. No settings can fix email sent to people who didn't ask for it, like bought lists or mass emails.
See what's wrong with your email, free
Type in your business's web address (the part after the @ in your email). You'll get a plain-English answer: what's working, what isn't, and what to do next.
Common questions
How long until I can send again?
Microsoft says restrictions are usually removed within an hour of being unblocked, and no more than 24 hours.
Can I unblock myself?
Only if you're an admin on your Microsoft 365 account with the right permissions. Otherwise, whoever manages your Microsoft 365 has to do it.
Can I still receive email while I'm blocked?
Yes. Only sending is blocked.
Does this mean I was hacked?
Not always, but Microsoft treats it as a warning sign. Check your Sent folder and inbox rules, then change your password and turn on two-step sign-in.
For your IT person or web developer
You can forward this page as it is. Everything below is written so someone who doesn't work with email settings every day can follow it.
The mailbox exceeded outbound limits or outbound spam policy and was added to Restricted entities (NDR 550 5.1.8 Access denied, bad outbound sender). Microsoft treats this as an indicator of compromise.
1. Check for compromise first
Look for unknown inbox rules (forwarding, moving mail to Notes, Junk or RSS), new external forwarding, changed signatures, unknown MFA methods, consented apps and admin roles. Reset the password (don't send it by email), require MFA and revoke sessions. Microsoft's guide is “Responding to a compromised email account”.
2. Unblock the mailbox
security.microsoft.com → Email & collaboration → Review → Restricted entities (direct: security.microsoft.com/restrictedentities). Select the user, click Unblock, review the recommendations, then Next, Submit and Yes. Requires Organization Management or Security Administrator. PowerShell: Remove-BlockedSenderAddress.
3. Wait and test
Restrictions usually lift within an hour, and within 24 hours at most. Send a test message to an outside address.
4. Prevent a repeat
Exchange Online limits: 10,000 recipients per day, 30 messages per minute per mailbox. Move bulk mail to a dedicated provider. The default alert policy “User restricted from sending email” notifies admins next time.
Related guides
Written by the SenderFix team. Last updated .