Interesting you mention Symantec. My guess is the users temp folder is a disaster area and therefore file access is slowed. so when the anti virus tries to scan the new temp file it locks it and takes to long so the send turns into a mess. Create an exception in the anti virus product for nstmp.htm in the temp folder.

We also recommend that the mail and imap mail folders in Thunderbird be exempted from any scanning while Thunderbird is active. The only contents of those folders are text versions of emails so they are non executable, but the files can be very large, resulting in contention issues between the ant virus and Thunderbird. Likewise the global-messages-db.sqlite should also be excluded from scanning. It is a large database file and as such is updated very frequently and is also very large. Again not a good file for invasive software like anti virus programs to be sniffing around all the time.