I have seen this in 5.6 (and earlier), 6.01 and 6.5. I had a number of queries in this forum several years ago and gave up on a real fix after no response. I lost one client because of it, two others have changed to MS/exchange.
After analysis it only seems to occur when there is a large CC or BCC list. If to single addresses it is ok. My fix was to use mercury (from
www.pmail.com) on a local PC which will take a cc/bcc list and on-send to single addresses only. Cheap/quick/easy and works even if a second grade soluation.
Regards