Sudden failure to send emails through specified SMTP server with python script

Found this thread because we're also suddenly encountering the same error:

"550 5.2.252 Sender throttled due to continuous send as denied errors."

We've been sending automated emails from a Filemaker database using a plugin for years now, but it started throwing this error within the last week. We routinely use a ReplyTo email which differs from the sending account, but we also normally specify the "Send as permissions" permissions for each account as well.

We however forgot to set the "Send as permissions" recently for a ReplyTo account, so a working theory is that this tripped the throttling, and perhaps if/when the throttling timeout expires (anyone know how long that takes?) - and now that we've set the permission for the offending account - perhaps things will work reliably again.

We are getting this on all our customer sites sending mail from systems using SMTP but getting no error but that could be the not so great SMTP implementation not being able to handle the error. If we manually restart the process sending the mail then it starts sending again.

We have asked some of the friendlier ones to log tickets with MS to see what the issue is but it feels like a new rate limit or an existing rate limit that is now being hard enforced.

I can't believe that there is no any official response from MS about this issue, consider it affects multiple customers already. I suggest everyone who has this issue open a ticket with MS.

I am still waiting for the reply from the MS tech. This already had a big impact to our business as we can't send automated emails to customers. We are thinking of using other SMTP provider to do the email relay.

Very frustrated with MS!

I found this article and looks like some recent changes in sending limits.

https://docs.microsoft.com/en-us/exchange/troubleshoot/send-emails/smtp-submission-improvements#new-throttling-limit-for-concurrent-connections-that-submitmessages

Thank you Frank.

I think this is somewhat related to our issues. However, from the article the error message should be "432 4.3.2 STOREDRV.ClientSubmit; sender thread limit exceeded", not "550 5.2.252 Sender throttled due to continuous send as denied errors.". And the behavior should be delay in email sending, not rejecting connection completely.

It could be when MS deploy this new throttle, they mess up something else, now we have an unexpected throttle.