The scanner.rb module inside of TMail fails to handle the '@' symbol in address and header checks.
The C version of scanner.rb handles these correctly.
This error only comes up when you are using the pure ruby version (which Rails does). If you are using the TMail gem, then it is not usually a problem as that usually uses the C version compiled in.
Handling is removing the @ symbol as possible ATOM character - this is per RFC 2822 Point 3.2.4
Patch is attached. You can only really see this work inside the TMail test suite as it fixes an internal TMail problem - I don't want to make a test case for TMail inside of ActionMailer.
You can see the error for yourself by checking out TMail revision 150 running "script/clobber/distclean" then "script/test" from the TMail trunk directory, then checking out HEAD and running the same.
Mikel Lindsaar
TMail maintainer.