Why does the 'arrow' reply icon sometimes disappear from the mailbox panel after sending a reply, or is not added at all? Sometimes I send a reply, and the purple arrow shows in the mailbox, and then disappears while I'm looking at it. Other times the arrow does not seem to be added at all. Sometimes it is added, and does stay. The only workaround I have is to reply for a second time (sending the reply to myself), before the purple arrow stays. I'm using IMAP mail, and compact the mailboxes fairly regularly (approx 250 messages in mailbox). I am running Thunderbird 38.6 on Win 10. The same thing happened on the previous few versions of T'bird too.