SMTP stands for Simple Mail Transfer Protocol. Email clients and sending services use it to submit messages to a server and relay them between mail servers. The receiving service decides whether an accepted message lands in the inbox.
How does mail transfer work?
The sending server tells the receiving server the envelope sender and recipient. It then submits the message, including its headers and body. The receiving server replies to each step. IMAP and POP3 typically come into play later, when an email client retrieves messages from a mailbox.
What do 4xx and 5xx mean?
A 4xx reply indicates a temporary problem. If a receiving server slows traffic because it is busy, for example, the sending server can queue the message and try again later. A 5xx reply indicates a permanent rejection of that attempt. If the recipient address is invalid, check it before sending again.
What can you check when sending fails?
If the send log shows a rejection, read the reply code together with the response text. If the message was accepted but arrived late, the email header can help trace its route.
