SquirrelMail 1.4.15 does not wait properly for "untagged responses" as described in RFC 3501 (IMAP 4rev1) chapter 2.2.2 in response to an APPEND command. This causes under certain circumstances SquirrelMail to display an error message: "FEHLER: Konnte die E-Mail nicht anhängen an INBOX.Sent. Server antwortete: RED)" (untranslated: "ERROR: Could not append message to INBOX.Sent. Server responded: RED)")
This has been reported by other user too:
With the help of a TCP stream dump I could track this down to the function sqimap_append_checkresponse() in functions/imap_general.php: In case the server sends an "untagged response" as described in RFC 3501 chapter 2.2.2 (i.e. a response that starts with "* ") SquirrelMail is unable to parse it.
The attached patch fixes the problem by a simple check for "untagged responses". A even better fix would wait explicitly for the response with the correct session id. But this patch at least doesn't make things worse. :-)