Unable to send email via smtp.office365.com

Hi, You could take these into consideration: Please check and turn off the firewall or any other scanning software temporarily. Make sure the sending server's IP is not on an SMTP block list. Verify that the receiving server is configured to use TLS. Check if the receiving server is configured to only respond to SMTP (not ESMTP) commands. TLS is part of the ESMTP protocol.

Unable to send email - Microsoft Q&A

Accepted answer

Tasadduq Burney 8,361 Reputation points MVP

2021-09-03T07:38:29.77+00:00
Hi,

You could take these into consideration:

Please check and turn off the firewall or any other scanning software temporarily.

Make sure the sending server's IP is not on an SMTP block list.

Verify that the receiving server is configured to use TLS.

Check if the receiving server is configured to only respond to SMTP (not ESMTP) commands. TLS is part of the ESMTP protocol.
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Xzsssss 8,861 Reputation points Microsoft Vendor

2021-09-03T08:20:37.65+00:00

Hi @JustinMicheal-7973 ,

Like Taz said, see if the firewall or the anti-virus softwares have any affect on this.

And in addition, see if these thread has any help:
http://azuredummies.com/2016/06/21/451-5-7-3-starttls-is-required-to-send-mail-office-365/https://www.moh10ly.com/emails-between-o365-and-on-premises-do-not-work/
Please Note: Since the web site is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.

Best regards,
Lou

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Xzsssss 8,861 Reputation points Microsoft Vendor

2021-09-06T08:30:29.713+00:00

Hi @JustinMicheal-7973 ,

Do the suggestions above help? If the issue has been resolved, please click “Accept as answer” to mark the helpful reply as an answer, this will make answer searching in the forum easier and be beneficial to other community members as well.

If you are still stuck in this issue, please feel free to post your questions.

Regards,
Lou

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Unable to send email

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