What is the best way to organize a backup mail server?

Question

FreeBSD

Hello everybody! There is a configured Exim mail server. Employees work through MS Outlook. Everything is working. I want to organize a reserve mailer, so that when the Internet channel No. 1 falls (through which the main mailer works), the sending goes through the backup mailer (there is Internet channel No. 2).
I already configured DNS. But I do not fully understand how best to organize it. I advise a lot of things on the network (especially DRDB), but can someone tell me how to do it beautifully?

Answer 1

Two static ("white") IP addresses on different channels. Both of them have different priorities in the MX DNS record. Everything will work the way you want.

Answer 2

Damn, you talked to the previous commentator there :-)
To reserve a shipment, you need two mail relays and a configured VRRP / CARP between them. That is, a virtual IP rises on the hosts, which will move to the second host if the first one falls.
For receiving, this will also work if you have two physically different channels from the same provider. Those. if you can broadcast with one IP on different channels. But it would probably be more logical to connect 2 hosts to two different providers, to different IPs, and make two MX records in DNS with different priorities (10 and 20). If the 10th is not available, senders outside will send mail to the 20th.
And yes, do not forget that in addition to sending mail outside and receiving mail from outside, there is also the task of distributing mail to users :-) That is, even if you have two SMTP servers, they will be able to receive mail almost guaranteed (with proper configuration ;- ) ), then they also need to be guaranteed to be able to put it on the server from which users are no less guaranteed to be able to pick it up ;-) Which leads us to a scheme of either 4 servers (two SMTP and two IMAP / POP3), or three servers - two SMTP, which also distributes mail via IMAP / POP3, and one mail storage server connected to both servers as a shared storage.

Answer 3

It is necessary to make the reserve e-mail server on VDS. On my website, I talked about this in detail https://bafista.ru/rezervnyj-pochtovyj-server-postfix/