Symptoms
When the value of the MessageRetrievalMimeFormat parameter is set to 1 or HtmlOnly by running the Set-IMAPSettings cmdlet, IMAP clients don't receive HTML messages for plain-text only emails in a Microsoft Exchange Server 2013 or Exchange Server 2016 environment.
Cause
This issue occurs because the MessageRetrievalMimeFormat parameter isn't honored during the Mime Convert process for plain-text only email messages.
Resolution
To fix this issue, install one of the following cumulative updates:
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Cumulative Update 6 for Exchange Server 2016 or a later cumulative update for Exchange Server 2016
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Cumulative Update 17 or a later cumulative update for Exchange Server 2013
Status
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
References
Learn about the terminology that Microsoft uses to describe software updates.
