Sending Bulk emails bounce bakc as spam

Hi Craig,

Good day. Sorry to hear this. That error 554 5.7.0 / 550 5.7.520 AS(4810) means Microsoft has detected something in the message or sending activity as potential spam. Since this can also happen when sending to only one person. The issue may not be related to the number of recipients.

Try to check the following:

1. Send a test from Outlook on the web. Sign in to Outlook on the web and send a short message to one trusted contact.
2. Send a completely new message with only a few lines of text. Don't include links, images, attachments, or your signature.
3. If the same error appears, stop retrying for a while. Repeated attempts can sometimes trigger additional anti-spam protection.
4. If you're regularly sending bulk emails to customers, it's better to use a dedicated bulk-email service, as Microsoft 365 mailboxes aren't intended for large-scale marketing emails.

If even a simple one-to-one email is blocked with the same error, the account may have an outbound sending restriction. In that case, Microsoft Support may need to review the account's sending status.

I hope this helps.

Regards,
Jhun

Hi,

Thanks for the information, but I have tried that. The only way I could get emails to send was to put different subject information. It does'n matter what the email body says either. It either goes through or it doesnt.

I also had to limit the amount of people I am sending the same email to. So about 4 is ok, but with no subject heading or a different one. and even then it might bounce back.

I woudl like MIcrosoft to check the sending staus as I don't beleive the fault is form my end. This has been going on since around 1 week ago.

Hi Craig,

Sorry i'm not able to help solve the issue. But thanks for confirming. Since you've already tested different subjects, message content, and recipient numbers, and the result changes depending on the subject or number of recipients, this may be related to how the sending activity is being flagged rather than the actual email content. Perhaps i could just give you some recommendations by trying the following:

1. Check your Sent Items for any unusual or unexpected messages. If you see anything you didn't send, change your password and review your account security immediately.
2. Check your account's recent sign-in activity for anything unfamiliar. An unusual sign-in can sometimes lead to temporary sending restrictions.
3. Avoid repeated attempts for now. Since some messages are going through and others are being blocked, repeatedly resending the same message may cause the filtering to become more restrictive.
4. If you need to send the same message to several customers, try sending individually with a short delay between messages, rather than sending to several recipients at once.

If the problem continues after these checks, it may be a temporary server side spam restriction that needs time to clear. Since this has been happening for about a week, I'd also recommend keeping a copy of the bounce-back message and noting which subjects or recipient counts are being blocked. That information can help identify the pattern.

Regards,
Jhun

Hi Craig,

With your email situation , at this point I think it's reasonable to suspect an account-side or server side filtering change rather than the message itself. Since this started suddenly last week and the same error can happen even when sending to just one person, it may be something related to the account or Microsoft's spam filtering. You can direct the issue to this site https://support.microsoft.com/en-us/support/. Point out the problem in full details.

Regards,
Jhun