thanks for the reply.
that's what I thought, but unfortunately this is not my experience (Rebex Securemail build 6198).
To check this, I set up a DNS server on a windows 2012R2 machine, blocked any external DNS requests by a firewall. in this situation, when using the DNS server, nslookup always reports a timeout.
when calling Smtp.ResolveDomainMx for the first time (after a ipconfig /flushdns), a SmtpException with message "socket error 11002 occurred", status "DomainResolutionfailure". From the 2nd call on, sometimes I just get back an empty array, no SmtpException gets thrown, sometimes another SmtpException with status DomainResolutionFailure.
The SmtpResolveDomainMx call always takes 20sec, the timeout parameter passed to the call.