A picture showing what happends when sending an email

The following picture shows the process of "user01@domain1.com" sending an email to "user02@domain2.com".

• In the good old days, mail servers were open-relay which accept any MUA without authentication. In that case, user01@domain1.com could connect to mail.domaon1.com directly. But that has not been the case for a long time due to security issues.

• An MTA can act as a relay host, which just helps pass the email to other MTA. 

• To avoid spam, most well-known mail servers only trust/accept emails from well-know mail servers. So it's hard to set up a new test-only mail server and use it to send email to well-known email servers. The common workaround is to use a well-known mail server as a middle-man (relay host) for our own test-only mail server.

•  e.g Gmail server can play as a relay host as long as we have a valid Gmail account. But the Gmail relay host changes the "from" address to the bound Gmail account's email address before sending out any relayed emails.