The entries are not always the same format so the header I've listed may not always apply.
Month date time year application process hostname/ip hostid ? ? task ENC ? MsgID Response-Code Sub-code Message Jan 13 10:45:47 2014 barracuda outbound/smtp: 127.0.0.1 1389609941-0669804e2e87d90001-ydWOdL 0 0 SEND ENC 1 461EB3A4A0D 250 2.0.0 OK 1389609947 el2si7258800wid.74 - gsmtp
C:\> telnet some_mailserver.net 25 SERVER SAYS: 220 some_mailserver.net ESMTP Sendmail 8.13.8/8.13.8; Thu, 23 Apr 2009 12:34:07 +0100 YOU SAY: helo localhost.localdomain SERVER SAYS: 250 some_mailserver..net Hello ???? [???.???.???.???] (may be forged), pleased to meet you YOU SAY: MAIL From:email@example.com SERVER SAYS: 250 2.1.0 firstname.lastname@example.org... Sender ok YOU SAY: RCPT To:email@example.com SERVER SAYS: 250 recipient <firstname.lastname@example.org> OK YOU SAY: DATA SERVER SAYS: 354 enter mail, end with line containing only "." YOU SAY: Hello Fred, can you call me? YOU SAY: . SERVER SAYS: 250 message sent YOU SAY: QUIT SERVER SAYS: 221 goodbye
From what I've seen, while transferring emails with a large attachment the connection to the receiving server will sometimes time out before the transfer is complete. This is dependent on the upload speed you receive from your ISP.
If it times out the transfer then needs to be restarted all over again. The appliance will only transfer 1 or 2 messages at the same time. So if a sender copies 10 people with a large attachment the mail is queued 10 separate times to transfer to each recipient. This can hold up the other messages in the queue resulting in the high out-queue alert.
I think the setting for this timeout is under the Advanced tab and then the "Remote IMAP/POP" settings.
I'm not 100% sure if this is the right setting but I can't find an outbound timeout anywhere else. Try raising the timeout there (ours is set to 320) and see if it has any effect.
Setting it particularly high is not recommended but I can't recall the exact reason why.