On Monday 21 March 2005 11:05, Menno van Bennekom typed: > > Clever trick. Do legitimate MTAs try to send to the second > > highest MXer if the primary is down? If so a fake third MX > > (even to a completely unused IP?) may have little downside. > > > > I.e. > > > > @ IN MX 5 realprimary.domain.com > > @ IN MX 10 realbackup.domain.com > > @ IN MX 20 fakebackup.domain.com > > > > Jeff C. > > AFAIK mailservers first try the highest prio, then the second highest > etcetera. > I once had a situation where both the primary and the secondary were down, > but still mail to us didn't bounce, old mails just started streaming in > when the servers came up. Somehow the mail-protocol is quite robust, I'm > not worried about using a 'fake' third MX. > Menno
Correct. SMTP is a (sort of) store-and-forward protocol. If I send a message to you, my mail server stores the message, does the appropriate lookups and tries to forward on. If the first MX fails (5), it'll try the next MX (10). If the next one fails, it'll try the third (20). If that fails, it stores the message and flags it for a retry n minutes/hours/days later. If the message cannot be delivered after y days (hours in some cases), the server generates a DSN for 'could not deliver the mail' and sends it to me. My mail server may not talk to yours directly either. In the case of my personal account, my postfix installation hands off to my ISP server for relay work. If the ISP server is unable to deliver, it has to generate the DSN - my box is no longer responsible.
