The SMTP mail system returned the following error: "Failure sending mail. Unable to read data from the transport connection: net_io_connectionclosed

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Posted on by Neha Sharma

Microsoft Dynamics NAV
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The SMTP mail system returned the following error: "Failure sending mail.
Unable to read data from the transport connection: net_io_connectionclosed.".
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OK
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We are getting this while using office 356 email, gmail with enable of less secure apps and custom domian. non of them is working. Can anyonce help us in that issue how to fix it.

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Gearoid on at

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Hi Neha,

This may be caused by not having TLS 1.2 enabled for your NAV 2016 environment. You can enable it by adding the following key to the relevant CustomSettings.config file for the NAV Service:

<add key="SecurityProtocol" value="Tls12" />

You need to have cumulative update 31 (or a later CU) for NAV 2016 to use the above option. See following KB article for more details: https://support.microsoft.com/en-gb/help/4171918

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