This browser is no longer supported.
Upgrade to Microsoft Edge to take advantage of the latest features, security updates, and technical support.
![](data:image/svg+xml, %3Csvg xmlns='http://www.w3.org/2000/svg' height='64' class='font-weight-bold' style='font: 600 30.11764705882353px "SegoeUI", Arial' width='64'%3E%3Ccircle fill='hsl(128, 8%, 22%)' cx='32' cy='32' r='32' /%3E%3Ctext x='50%25' y='55%25' dominant-baseline='middle' text-anchor='middle' fill='%23FFF' %3EA%3C/text%3E%3C/svg%3E)
Hi,
We have been sending automated emails through a python script for a while now and suddenly getting this error:
Failed to send an E-mail through the specified SMTP server [smtp.office365.com]:
Mailbox unavailable. The server response was: 5.2.252 Sender throttled due to continuous send as denied errors.
[Hostname=BN9PR12MB5179.namprd12.prod.outlook.com]
All of the login/password information is correct in the python script for sending the email, so why is this suddenly happening?
Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Locked Question. This question was migrated from the Microsoft Support Community. You can vote on whether it's helpful, but you can't add comments or replies or follow the question.
I can verify that we are not having the issue anymore as well. Nothing was changed other than calling microsoft multiple times to complain and get help with it.
I can verify that we have not had this error crop up again since Tuesday after switching our license for our sending account. I don't feel confident that was really a fix but probably more of a reset. But maybe Microsoft has changed something so it won't error out again? I wish I had more information.
Some good news here. For the first time in the past week, we don't have the error message when sending out email from applications. I am not sure if this is because MS fixed the issue or we haven't reach the threshold yet. I will keep monitoring this and report back here.
Can someone please verify if this got fixed for you as well?
Thanks
This is causing major issues and Microsoft won't get back to us. Can you please resolve this? I spent hours and hours looking at this for a few days before I saw this thread.
We have been having this issue all week.
On our UK customer sites, our Service application that has been running flawlessly for months now times out when sending via SMTP. Restarting the service temporarily fixes the issue but it reoccurs a few times every day. This is only affecting Office 365 SMTP servers.
For us, login fails with a 535 error.