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How to synchronize mail servers?

I was wondering how do you set up multiple email servers where all the emails available on one server are available on the others?

Like in the event that a server goes down how do you ensure that clients can still access their mail?

Do you synchronize email servers?

How do you ensure that a client can access mail if they are connected to mail1.example.com and mail1 goes down?
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You use an NFS share as the repository into which both write. the mailervers are only head units.
This requires the mail servers to function in Maildir storage format, one message per file which will avoid contention if using mbox format with each trying to get exclusive right to a users /var/spool/mail/$USERNAME to write the newly received message.

If you do not want a single storage type of setup, the only other way is to duplicate the storage and use rsync to copy the data from one to the other. in this case only one will be operating at one time in the form of receiving incoming connections.
no load distribution that is available with a common storage.

If one goes down, you have the possible loss of data between when it was received and the next event of rsync copy/mirror.
The Data on the mail server, queue, etc. are transient. often with all things considered, a message submitted by a user should be out in less than a minute. unless there are issues with the remote or this is undeliverables.

With the above said, what resources are available to you and what are you considering.
Backups should always exist.
burnedfacelessAuthor Commented:
Can you recommend some books or resources for me to get started?

I'm a Cyber Security student and I haven't gotten to my major courses yet.

There don't seem to be any courses on Udemy and some of the books look out of date.
Not sure what you are asking.

Mail server postfix, exim, sendmail, qmail, need a storage where the messages are aggregated within the user.
The mail servers have access to the same user base, I.e. Using some centralized user management, nis, LDAP, MySQL backend with all servers
The mail servers then mount the same user storage area I.r. /home/users/

Large environments have incoming mail handlers.
They then have setup head units that provide users access to their messages
And a set that deal with user outgoing messages.

The scale if your interest ..

Redundancy at times is setup through the use of a backup mailserver which stores messages for your domain with the goal of delivering them to the primary when it recovers and which will distribute these mailings to their final destination.

Consider the backup as the overflow handler when the primary is unavailable.
The backup only validates the recipient is on the domain, it validates nothing beyond that.
burnedfacelessAuthor Commented:
That actually helped a lot.

Do you think it would be important for me to study this in depth as someone who wants to specialize in internet security?
not particularly this is more of a scaling and load distribution versus security.

knowing how to secure a mail server, configure/setup anti-virus/spam, etc. would be worth while, but a configuration on one can be replicated to others... for scaling purposes.
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