When sending mail to certain domains (mostly yahoo.com), the message may be delayed for an hour or more (sometimes alot more).
The messages show up in the smtp que immediately and the que folder state is "active". If I do a find on the messages in the folder, their state is "queued". They will eventually be delivered.
This seems to be an intermittent problem so I suspect external problems but have had a few other domains recently with the same problem.
I have done the telnet test from the Mail Front-End server to the problem domains mx servers and it works fine.
Searches for solutions has taken me to antivirus (Trendmicro), EDNS (turned off), reverse dns entries, etc...
I did a packet sniff on the server segment and it doesn't seem to be trying to send out the messages even though everything looks like it should be (que active and messages queued).
Thanks for any help you can give.