Take the 2-minute tour ×  CentOS 5.x | SendMail  This is
somewhat related to my other question about round robin but as
it's more general, I thought I'd separate it out.  If SendMail
is smarthosting to a FQDN with multiple A records and it can't
reach the first host it tries, will it immediately retry
connecting to one of the others? Or will it defer the message
and try again during the standard queue runner interval?
share|improve this question  1 Answer 1  up vote 2 down vote
accepted  The bat book says:  As usual, trying a host means
trying its MX records first, or its A or AAAA record if there
are no MX records   When sendmail finds multiple A or AAAA
records for a host (and no MX records), it tries them in the
order returned by DNS, but looks up and uses AAAA before A
records. If sortlist is specified in the /etc/resolv.conf file,
DNS returns the A or AAAA record that is on the same network
first. The sendmail program assumes that DNS returns addresses
in a useful order. If the address that sendmail alw