We had a similar problem a little while back where random emails were not arriving. At the time the DB was set up to send email through SMTP. It turned out that when using the SMTP to send, some destinations such as Hotmail would reject the email as spam or put it straight into the junk folder. We had to switch over to client based sending for this reason.
This is a known issue with Send Mail.
For More Information see: Email header malformed when using SendMail
This is one of many acknowledged bugs that can be found in the Known Bugs List here in the Report an Issue section of the forum.
It can also be downloaded as a database file from: http://www.4shared.com/file/8orL8apk/FMP_Bugs.html