"Reply all" emails I am sending are hung up in the outbox. They also show up in the "unread" section of local folders.

[Harry_Flashman_Jr]

Hung up in outbox.

[Mark_Wentzel]

I am having a similar problem. If I “reply all” to a received email (one recipient in CC), then message get hung up in outbox. Other emails with reply to a single sender get sent without problems.