Email – Is Sendmail an SMTP Server? Differences Between MTA and Mail Server

centosemailsendmail

I am new to mail and do not exactly know how it works.

I have seen administrators use sendmail to send out email. Is sendmail an smtp server or is it a client that relies on smtp server to relay email? I googled and read that it is a mail transfer agent.

Is there any difference between a mail transfer agent and a mail server?

Assuming there are no security checks (reverse dns etc) in place when I use sendmail to send out an email to user@yahoo.com: Do I still need to go through another smtp server (google.com?) or can I send mails from my server using sendmail?

It will also be good if some kind soul can point me to a beginner guide on how mail servers work.

Thanks

Best Answer

IMHO this tiny tutorial, MUAs, MTAs, and MDAs, is very good at explaining the basic concepts of the Unix email system.

Yes, properly configured sendmail should be able to send email to any MTA on reachable hosts (but that MTA may refuse the mail for various reasons)

Now when you say mail server that typically refers to an alternate path on either/both of:

  • Receiving side
    Basically replacing the destination host MTA+MDA (and the user mailbox) with a mail server + various mail client(s) running on various separate machines model (like web/POP/IMAP/etc, typically applicable to most public mail services like gmail/yahoo).
  • Transmit side
    Basically replacing the sending host MUA+MTA with the mail server + various mail client(s) running on various separate machines model
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