I get the impression from messages posted here in the forum that SMTP is slightly harder to get to work correctly. There's also a bug in SMTP emailing that can cause issues for you.
For More Information see: Email header malformed when using SendMail
This is one of many acknowledged bugs that can be found in the Known Bug 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
Thanks PhilModJunk, that was most helpful. Now the next question is, how do keep from having to click "allow" for the security in Microsoft Outlook when I'm sending multiple emails at one time?