Hello,
I apologize for that mistake. What versions of Thunderbird and gContactSync are you using?
Also, could you enable verbose logging? You can do this in the Address Book window under gContactSync -> Preferences -> Interface and check
Enable verbose logging. That will add more information to a log that gContactSync keeps to help me figure out why it is replacing your e-mail addresses.
If it happens again could you e-mail me the log? You can do that from
this form or over e-mail to joshgeenen [at] gmail <dot>com. Before you send the e-mail you can remove irrelevant parts of the log (but please keep the top version information and preferences) and replace real info with fake info.
Josh