Since the 2012-01-05 version (that I'm aware of) when replying to messages that are to individuals that have email addresses on the server Squirrel is located on the reply to address puts the @ symbol followed by the name of the mail server. This is to all users that have left the reply field blank in their preferences which by default this is the way email addresses are setup. If the reply to field is added the problem goes away. Again this is for all users that reside on that same server Squirrel was setup for (the mail server in reference). Replys to other users is fine. Just those all sharing the same server that Squirrel is setup on.