Mysterious Reply-To address

After quite awhile, I've returned to Yanoff and am stumped by the 
Reply-To address that's attached to emails sent via Yanoff.

In Server-Prefs, any email address I enter in the Reply-To: field of 
the Mailserver, gets characters added.  For example, if I enter the 
address, [email protected] the email arrives with the address, 
[email protected]   Also, if I enter only xx in the Reply-To: 
field of the Mailserver, the email arrives with the address 
[email protected]

To add further confusion, if I leave the Reply-To: field of the 
Mailserver blank, the email arrives with *only* my name exactly as 
it is entered in the Realname: field (as shown in the >> page of the 
Server Preferences).

I cannot find where Isn.net and/or mx1.Isn.net is coming from.

Les Blalock 
Tungsten T