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

Anonymous
2021-07-15T20:57:04+00:00

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 and Office | Subscription, account, billing | For business | Other

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.

0 comments

43 answers

Sort by: Newest
Most helpful Newest Oldest
  1. Anonymous
    2021-07-20T16:13:47+00:00

    This started happening for us July 15th. We use an IIS SMTP server for sending out automated invoices, purchase orders and other emails that IIS relays to Microsoft 365. Has been working great until last week. Sometimes emails get sent out but other times emails get dumped in the badmail queue on the SMTP server. And they won't resend.

    Action: failed

    Status: 5.2.252

    Diagnostic-Code: smtp;550 5.2.252 Sender throttled due to continuous send as denied errors. [Hostname=CH2PR12MB4310.namprd12.prod.outlook.com

    Need to find a fix for this asap..

    Was this answer helpful?

    0 comments
  2. Anonymous
    2021-07-20T15:17:27+00:00

    This seems to be the only thread related to this particular error that I can find online. I am assuming there is some new throttling feature in Exchange Online that blocks an account? I'd love to figure out how to disable this for one account that we use to send a high volume of emails.

    Was this answer helpful?

    0 comments
  3. Anonymous
    2021-07-20T15:00:58+00:00

    I also noticed from the audit log that the emails sent by those accounts are "SendAs". But the user and SendAs user are the same email address, why those emails are treated as "SendAs" emails?

    TimeStamp : 2021-07-20T10:48:46

    Action    : SendAs

    User      : ******@domain.com

    SendAs    : ******@domain.com

    Status    : Succeeded

    SentBy    : ******@domain.com

    TimeStamp : 2021-07-20T10:48:46

    Action    : SendAs

    User      : ******@domain.com

    SendAs    : ******@domain.com

    Status    : Succeeded

    SentBy    : ******@domain.com

    TimeStamp : 2021-07-20T13:44:30

    Action    : SendAs

    User      : ******@domain.com

    SendAs    : ******@domain.com

    Status    : Succeeded

    SentBy    : ******@domain.com

    TimeStamp : 2021-07-20T13:44:30

    Action    : SendAs

    User      : ******@domain.com

    SendAs    : ******@domain.com

    Status    : Succeeded

    SentBy    : ******@domain.com

    Was this answer helpful?

    0 comments
  4. Anonymous
    2021-07-20T13:14:24+00:00

    We are facing the same issue. Couple of mailboxes we are using for the scan to Email or notification from D365, start to get this error after sending out some emails. When this happens, we have to wait for hours, then it starts to work for a couple of hours.

    Here is the error message:" Mailbox unavailable. The server response was: 5.2.252 Sender throttled due to continuous send as 

    denied errors. [Hostname=BL0PR12MB5012.namprd12.prod.outlook.com]

    Looks like there is some mechanism in o365 to throttle those emails. I have opened a support ticket with MS, but no luck yet.

    Any help?

    Thanks

    Was this answer helpful?

    0 comments
  5. Anonymous
    2021-07-20T10:39:44+00:00

    Am Seeing the same issue. By any chance are you using IIS SMTP server to replay messages?

    Was this answer helpful?

    0 comments