To get to this page, to to Mail Connectors → (choose Mail Sender) → Settings (if the Sender uses the 'SMTP Direct' method).
When VPOP3 is sending using the "SMTP Direct" method, it has to determine where to send outgoing messages. It does this by querying the 'MX' DNS records for the destination domain. So, it needs to access a DNS server to do this.
The DNS Servers to use box lets you specify DNS servers which VPOP3 should use. If you leave this blank, then VPOP3 will use the DNS servers configured in the Misc Settings. In some cases you may wish to specify different DNS servers here for reliability or other reasons.
The DNS Overrides button lets you specify fake DNS entries which VPOP3 will use in some cases. There are many uses for this facility, for instance:
•You may find that a particular recipient domain will not accept mail from you for some reason. In this case, you may wish to tell VPOP3 to use an SMTP relay server somewhere else (e.g. at your ISP) for sending to that domain.
•You can tell VPOP3 to send through a different mail server if it has failed to send messages for some time
•You can tell VPOP3 to send big messages through a different mail server
•You can tell VPOP3 to send messages from a particular user through a different mail server
•And so on.
VPOP3 remembers (caches) DNS results so it does not need to look up the DNS results again if it sends several message to the same domain. The DNS cache size setting tells VPOP3 the maximum number of cached results to keep. (It may keep fewer than this depending on how long the recipient specifies that DNS entries should be cached, and how often it sends messages).
The MX sending threads option tells VPOP3 how many messages it should send at the same time. Because VPOP3 may be sending each message to a different mail server, there can be some time while VPOP3 is negotiating the connection, so it helps performance if VPOP3 processes several messages at once. The default of 10 is a reasonable amount in most cases.
Remember that if you send a single message and list many recipients for that message in different domains, then, when using SMTP Direct sending, VPOP3 will have to send the message several times, once to each target domain.