What is the difference between a MailServer and a MailGateway?
migrated from stackoverflow.com Apr 4 '11 at 19:51
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Nothing really. They are both generally used as terms to describe a 'Mail Transfer Agent' or MTA.
Now, in some implementations you'll see Mail Gateway used to describe a device that sits on the edge of your network, accepting mail, and running security scans against it. But really this is just another form of MTA running anti-virus and anti-spam software.
Some other implementations will define a 'Mail Server' as a server that accepts mail and queues it to be delivered to the end user, which is yet another type of MTA.
And finally you'll find implementations that use the terms inter-changeably.
The common thread is that all of the above are MTA's.
As per Zypher's answer, it's just terminology and is used quite inconsistently. However, in general a "mail server" (two words, not one) is the system that actually handles the mailboxes.
A "mail gateway" is a mail server that receives the mail but passes it on. e.g. Some companies provides store and forward services, which are great for those who for one reason or another may not always have their mail server on-line. The store and forward system is a mail gateway. Similarly, a hosted spam filter is also a mail gateway because it receives the mail on your behalf and after processing it passes it on to the destination mail server.