Anomalous sending patterns were detected resulting in the user being restricted from sending mail

I’m using Microsoft Office 365 SMTP to send PDF invoices and it works like a charm until December. Microsoft blocks E-mail account with the error message:

Anomalous sending patterns were detected resulting in the user being restricted from sending mail.

I’m sending invoices in waves of 20 E-mails every 5 minutes and tried to create a new spam filtering in EOP without success.

Any ideas? Thanks in advance!

Hi David,

From the error it seems the emails are blocked on the outgoing email server. Your sending frequency is within O365 sending limits, but MS security can still treat certain emails as spam.
You can try to turn on/off various outbound spam policies (as you already tried). Collect also the Extended Message Trace, maybe it will tell you which policy blocks the emails.
There are three different techniques for sending email with SMTP. Try if the problem applies to all.

If that does not help, you should contact O365 support.

However for sending massive emails Docentric offers and alternative solution to use third-party massive email provider (ex. SendGrid) and set up in Additional email accounts to use that provider just for certain emails (based on sender address).

Thanks @MihaV , I’m in touch with O365 support but they are lost as me.
I’ll keep you updated.

Finally, Microsoft didn’t give us any solution and we’re using a third party E-mail send service…

Hi, David,

We are also facing same issue. Any solution you found on this part?

We are using office 365 email system and configured it as IMAP with our CRM ZOHO. When we send email to contact from ZOHO CRM, office 365 restrict user stating " suspicious email pattern is detected" we are in conversation with Microsoft but seems we are lost and they are not giving any solutions to this matter.

Any solution will be helpful as we are now nowhere due to this issue.

Hi Keshu,

The only solution I found was work with sendgrid. Sad but true…
It works like charm.