Which email services use SMTP

SMTP server: what is it actually?

If the recipient domain is connected to the same mail server as the sender, the email is delivered directly. If this is not the case, the MTA breaks them down into small data packets that are as short as possible and at the same time Lowest traffic route forwarded to the destination SMTP server. The packages sometimes pass several times MTAs on external SMTP servers (called "Relays" in technical jargon), which take care of the constant forwarding.

Incoming mail server of the recipient

Upon arrival at Destination SMTP server the data packets are reassembled into a complete e-mail. This is checked again for spam by the MSA and / or MTA and then stored in the Message store of the incoming mail server transfer. From there, the so-called “Mail Delivery Agent” (MDA) sends it to the recipient's e-mail. Then other network protocols - either IMAP or POP3 - take over downloading the electronic mail to the recipient's SMTP client.

From a purely technical point of view, it would also be possible to send emails directly from the sender's SMTP client to the recipient's client. The use of an SMTP server offers a clear advantage: If the recipient's incoming mail server is busy or temporarily out of order and the e-mail cannot be delivered, the responsible SMTP server tries automatically at regular intervalsto deliver the mail. This happens until the delivery succeeds or the mail is returned to the sender as undeliverable.