In simplest form an SMTP server is a computer that receives outgoing mail messages from users and routes them to their intended recipients. All SMTP servers implement some version of the Simple Mail Transport Protocol.
Electronic mail server software uses SMTP to send and receive mail messages, user-level client mail applications typically only use SMTP for sending messages to a mail server for relaying. SMTP is a “push” protocol that cannot “pull” messages from a remote server on demand. To retrieve messages only on demand, which is the most common requirement on a single-user computer, a mail client must use POP3 or IMAP. Here we have described a series of actions which happens when you hit Send button
- The request goes to mail server where it connects to its port 25. Then it sends some command for its communication and if evrything works fine then the message is posted in to the network.
- Server maintains a mail queue: it will save the mail locally and send it together with all the other queued mail in intervals (for example, every 30 minutes). This behavior depends on the configuration and the server may also deliver the mail instantly.
- To get the mail POP3 or some other protocol is used.