This routing method is generally used if you have a catch-all or 'multi-catch' mailbox with your Internet provider. With this method, VPOP3 will look at the To/Cc and other fields in the message headers to try to work out who the message is for. If the recipient is not listed in the headers (eg for BCCd mail) it may not be able to automatically detect who the message is for.
The Accepted Domains setting tells VPOP3 which addresses it should process in the headers, and which it should ignore. It cannot simply process all email addresses in the header, as a message may have been sent to you, and CCd to someone at a different company - so VPOP3 has to know to ignore the addresses for the other company.
In the Accepted Domains box, specify a list of your registered domains, separated by semi-colons (;). Do NOT put preceding @ characters.
If you only have a few email addresses in the domain, then you can specify those explicitly. (If you have most addresses in the domain, but a few are elsewhere, use *REMOTE mappings instead, and list domain name here)
*Configure Routing Options* This button takes you to an advanced settings window where you can specify 'tweaking' options for the message routing.
Disable default user → user mappings This setting is usually not set, but in some multi-company situations it can be useful:
Let's say you have two domains: domain1.com and domain2.com. Maybe those domains are owned by different companies sharing one VPOP3 server.
Then, you have two email addresses firstname.lastname@example.org and email@example.com, which go to different users.
You can create two users mark1 and mark2 and mappings of firstname.lastname@example.org → mark1 and email@example.com → mark2
All will work great like that.
HOWEVER, someone could send a message to firstname.lastname@example.org, and it would go to mark2, but it is addressed to a company where mark2 does not work (mark2 works at domain2 not domain1). So, this is confusing.
So, setting disable default user→user mappings means that email@example.com (and firstname.lastname@example.org) won't be recognised as valid addresses.
If you check this box, you have to create mappings for everyone. Without it, VPOP3 assumes a mapping of '<user>@<any recognised domain>' → '<user>'