Making an e-mail buffer, do I use POP3? [closed]

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Ok, I'm a bit confused with these mail protocols so I just have to ask. I'm making an app for a very special use case we have at work. We have two e-mail servers sending mail to eachother (two seperate domains). The problem is that one of the servers is frequently moved (the server is in the field, and has to be mobile). When that server is moved, it can't be connected to our network and therefore mail cannot be sendt to this server.

What I'm making开发者_StackOverflow中文版 is an app that will be between our two e-mail servers, intercepting the e-mail trafic that goes between them and, if one of the servers cannot be reached, my app should store the e-mails in a buffer, so that when it detects that the e-mail server is back online, it can safely send the e-mails the other server did not recieve due to it being offline. The e-mails in the buffer should also be able to be sendt to an alternative e-mail so they can be acted upon immidietly should the recieving e-mail server be down for longer than normal...

So, I've figured out that I need to use the SMTP-protocoll to send e-mail, but what do I use to recieve them? All examples I've read so far in C# and Python are about connecting to an allready established POP3/Imap4 server and recieve mail from there, and I think setting up my own POP3/IMAP4 server for intercepting mail before sending it on is doing it the really hard way...

So how do i recieve/intercept e-mail without the use of POP3/IMAP4?

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If I understand the question correctly you simply want to buffer your emails. You could be much better off either using a third party to act as a backup mail server or set up additional mail servers as lower priority servers to collect the mail should one of the servers not be reached. You can ask on ServerFault about setting something like this up. I can't see the real benefit of writing an app yourself.

If you do wish to write something yourself then you will most likely wish to write both an SMTP server and and SMTP client one to accept the emails and one to deliver the message.