Does Outlook use a TLS connection for transmitting email

Mark 0 Reputation points
2026-07-18T10:32:24.1066667+00:00

When sending an email to a medium sized group distribution, 50 recipients, those with gmail addresses are bounced. The error message suggests using a TSL connection but I cannot find any settings/options for this.

Exchange | Exchange Server | Other
Exchange | Exchange Server | Other

A robust email, calendaring, and collaboration platform developed by Microsoft, designed for enterprise-level communication and data management.Miscellaneous topics that do not fit into specific categories.

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  1. Hin-V 16,830 Reputation points Microsoft External Staff Moderator
    2026-07-18T17:04:58.8666667+00:00

    Hi @Mark

    Thank you for reaching out. 

    To ensure I fully understand your situation and provide the most effective assistance, I’ll need a few additional details, such as:  

    Just to clarify, could you please confirm whether your account is hosted and managed via Exchange Online, an on-premises Exchange Server, or another mail system? 

    Does the issue occur with all Gmail recipients or only specific Gmail addresses within the distribution list? 

    May I know the exact error code/NDR (bounce-back message) you receive? 

    Any update would be appreciated. 

    Please understand that our initial reply may not always immediately resolve the issue. However, with your help and more detailed information, we can work together to find a solution. 

    Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.  

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  2. Mark 0 Reputation points
    2026-07-27T15:06:55.6133333+00:00

    Hi HV

    Thanks for your response. Sorry for the slow reply, but here are the answers to your questions (I hope, I am no expert in this field and have only just taken over the Admin role for this site).

    I understand that our account is hosted and managed via Exchange Online, we certainly have no on-premises Exchange Server.

    All gmail addresses were bounced.

    Error messages are:

    Your message couldn't be delivered. Despite repeated attempts to contact the recipient's email system it didn't respond.

    Contact the recipient by some other means (by phone, for example) and ask them to tell their email admin that it appears that their email system isn't accepting connection requests from your email system. Give them the error details shown below. It's likely that the recipient's email admin is the only one who can fix this problem.

    For more information and tips to fix this issue see this article: https://go.microsoft.com/fwlink/?LinkId=389361.

    Diagnostic information for administrators:

    Generating server: [**[Moderator note: Personally Identifiable Information removed]**PROD.OUTLOOK.COM] Total retry attempts: 4

    Remote server returned '550 5.4.300 Message expired -> 421 4.7.28 Gmail has detected an unusual rate of mail originating from your DKIM;domain [Moderator note: Personally Identifiable Information removed]. To protect our users from;spam, mail sent from your domain has been temporarily rate limited.;For more information, go to; https://support.google.com/mail/?p=UnsolicitedRateLimitError to;review our Bulk Email Senders Guidelines. [Moderator note: Personally Identifiable Information removed]- gsmtp'

    Original message headers:

    Received: from 
Thread-Index: AQHdDSI0Wt/FKiZId0SOKZOe1ctYRg== Date: Mon, 6 Jul 2026 08:44:26 +0000 Message-ID: <

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