The SMTP Return Address (or Return Path) is a hidden part of a message which says where bounce messages (and other automated responses) should be sent. (It is not the same as the Reply address which is where your email client will send replies to messages).
By default, VPOP3 will send out messages using the same Return Address as was used when the message arrived at VPOP3 (even if the message was an incoming message which is now being forwarded). However, some ISPs will check the return address and reject messages if they do not recognise it. For this reason, you can tell VPOP3 to change the Return Address if necessary.
* The Use Default setting will make VPOP3 use the original return address. * The Always Set To setting will make VPOP3 use the specified email address on ALL outgoing messages sent via this Mail Sender * The If the original address is not local, set to setting will make VPOP3 check to see if the original return address is local. If it is, it will use that, otherwise it will use the specified email address instead.
If the default setting will not work, the usual setting to use is the If the original address is not local setting, and set the return address to something like postmaster@yourdomain.com
The SMTP standard has a very important rule, that if you do not want to receive bounce messages for your message, then you set the Return Address to a blank address (NULL). This stops bounce loops being set up where a bounce message bounces, and that bounce bounces again etc etc. As the Return Address is also often used for other automated responses such as delivery receipts, autoresponders etc, this is very important.
However, some ISPs don't conform to the SMTP standard and will block NULL return paths in the mistaken belief that they are insecure. In this case, you can tell VPOP3 to change NULL return paths to a specified email address here. Use something like no-one@yourdomain.com or another email address which will never, ever have an autoresponder, assistant, forward, etc on it.