SMTP stands for Simple Mail Transport Protocol. To put it simply, it's the language that mail servers and mail clients (e.g. Outlook, Outlook Express, Thunderbird) use to communicate with each other to deliver an email message from the sender through the internet to the recipient. To see how the process works, refer to the diagram below:



After you compose an email message, hit 'send', and the message leaves your mail client's outbox, its first stop is your outgoing SMTP server. This is the mail server that you have configured to use for sending all of your outgoing mail (e.g. smtp.ultrasmtp.com).

Your outgoing SMTP mail server then attempts to deliver the message to the recipient's incoming mail server. If everything goes well, the recipient's incoming mail server accepts delivery of the message. Finally, the recipient's mail client picks up the message from their incoming mail server, and the recipient receives the message.


Related:

Why do Legitimate (non-spam) Email Messages Sometime Not Reach Their Recipients