In Microsoft Exchange Server 2019 or Exchange Server 2016, consider the following scenario:

  • You send an email message that has an attached email message to User A.

  • The attached email has more than two recipients in the To line.

  • User A uses the mobile application that connects to Exchange Server by using Internet Message Access Protocol (IMAP).

  • User A can't download the email message and the mobile application may crash.

In this situation, when you run the FETCH (BODYSTRUCTURE) command of IMAP, you may find a space is inserted between each recipient. This violates RFC 3501. For example, there's a space between brackets:

(("user1" NIL "user1" "mydomain.local") ("user2" NIL "user2" "mydomain.local"))


To fix this issue, install one of the following updates:

For Exchange Server 2019, install the Cumulative Update 2 for Exchange Server 2019 or a later cumulative update for Exchange Server 2019.

For Exchange Server 2016, install the Cumulative Update 13 for Exchange Server 2016 or a later cumulative update for Exchange Server 2016.


Learn about the  terminology that Microsoft uses to describe software updates.

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