Emails to Microsoft 365 - 451 4.7.500 Server busy
Hi all,
Since the last email issues with Microsoft few months back (https://www.linode.com/community/questions/22287/550-57511-access-denied-banned-sender-office-365), we have setup a separate SMTP server out of Linode with other provider.
So last week we were testing to switch the SMTP back to Linode server. Again, emails to Microsoft are hit and miss, some emails to Microsoft are having the error below. It will take a long time to retry and send out
SMTP error from remote mail server after end of data: 451 4.7.500 Server busy. Please try again later from [***]. (S77719) [SG2APC01FT0013.eop-APC01.prod.protection.outlook.com]
It looks like emails from Linode to Microsoft is "rate-limited"?
We have switched back to external SMTP now and no problem with external SMTP.
Thanks
3 Replies
It looks like emails from Linode to Microsoft is "rate-limited"?
…or greylisted. Does your server try again after some amount of time & get a successful delivery? If not, I would try that first. Eventually, your mail will start to go through faster (without the delays) because M$ will have established your Linode's "reputation".
Greylisting is a common spam defense. It’s very simple to do…but very effective. I used it myself for a long time.
— sw
Thanks, that should be it
It will eventually be accepted by Microsoft, but some take hours of retries… our users are sending support tickets for emails not delivered immediately