Every time VPOP3 checks for remote MX server for sending messages using direct MX sending, it runs a script called mxserverrules.lua which can be created/edited in Settings → Scripts (or in the VPOP3 directory in VPOP3 v6.6 or earlier).
GetCheckCache(“<Domain>”, numRetries, messageSize)
returns true if VPOP3's DNS cache should be used, or false to do a fresh DNS lookup
GetOverrides(“<Domain>”, numRetries, messageSize)
returns DNS Server Map, continue
If continue is false, then VPOP3 will act as if there were no MX/A records defined for this destination
(VPOP3 7.10 and later)
CheckServer(“<Domain>”, “<MX Server>”, numRetries, messageSize, defaultResult)
returns true if the MX server is OK to use, false if it should be ignored
(VPOP3 8.6 and later)