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Outlook - 'bounced' emails using SMTP

Started by norman, Aug 03, 2022, 09:36 AM

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The symptoms are that not all your emails get sent when using SMTP under Microsoft Outlook. This will have been occurring since November, 2021. It may just affect the occasional email. It does not affect the sending by Outlook, only when sending emails using SMTP with Outlook settings.

We have been notified of the effect in Loadmaster and Sameday Express by our customers when using Microsoft Outlook. Other email clients are not affected, so if you use your own SMTP server or one like Google then you won't have had any problems.

You get a message similar to this :–

"421 4.7.66 TLS 1.0 and 1.1 are not supported. Please upgrade/update your client to support TLS 1.2. Visit"

A percentage of the emails are being deliberately 'bounced' by the Microsoft Outlook SMTP server as it believes they have been encrypted with TLS 1.0 or TLS 1.1. Often simply resending the same email will work on the second attempt.

This is an action taken by Microsoft in an attempt to force users to update their email client software to the latest secure version which  should use the TLS 1.2 format. Unfortunately it doesn't seem to be working as intended as it bounces their own emails.

There are workarounds.