I will deploy a web server, but I don't have much experience.
I would like to know what happens with mail that is send when the server is offline (restarting, maintenance, etc.).
When an internal user wants to send and receive mail, it's logic that he can't access the server. But when an external person uses his mail server to send a mail to my mail server, will the mail be resent on a later time?
The mail server will be a virtual machine, is this a good idea?
Thanks.
Best Answer
The SMTP protocol is very resilient in design. Most modern MTAs will attempt to resend a message every 4 hours for up to 5 days though this is configurable by the particular systems admin there is usually very little need to change from this.
The only server you would need to be concerned about would be the initial server you're connecting to in order to send the message. So long as this server accepts the message into it's queue it will continue to be processed until it can no longer be sent and then a undeliverable message will be sent back notifying you. Once it has been accepted by the initial MTA you're concerns about it's delivery should be negated regarding maintenance and restarts. Also given that many larger mail providers run clusters of mail servers it's likely that the maintenance and restarting of any one will affect mail transmission.