Question:> I am having problems with setting an autoresponder in VPOP3. We are getting an SMTP error which says domain invalid when it tries to send it. The logs show it has responded but it never goes out.
> Below is a screenshot of the error
Answer:This problem happens when using an ISP's SMTP relay server which doesn't conform to the complete SMTP standard. It generally happens with those not designed for business use.
What is happening is that they are validating the sender's email address in the SMTP envelope and checking that it matches your domain. The problem is that they are not allowing a blank sender address as the SMTP standard requires. A blank sender address is used when you don't want to receive bounce messages, which is the case with autoresponse messages (and most other automated messages), and is a required feature of SMTP.
Fortunately, VPOP3 supports a workaround for these broken ISPs. Go to Mail Connectors -> Mail Senders -> Return Path Settings, and in the 'Change NULL return addresses to' setting, enter a valid email address. This email address must be a local email address which will NEVER have an autoresponder, forwarding, assistant or any other such feature. If it does, and a bounce message is generated, it could lead to an infinite loop of error messages between VPOP3 and the remote server (which is precisely what the NULL return address is designed to prevent).
Also see: https://helpdesk.pscs.co.uk/890788-Sending-mail-failing-with-an-error-about-a-blank-sender