SMTP transports email from applications to mail servers and between mail servers. Port 25 is commonly used for server-to-server relay, while port 587 is typically used for authenticated message submission with STARTTLS.
Simple definition
SMTP is used to transmit an email from an application to a server, then between the mail servers.
Technical definition
Simple Mail Transfer Protocol (SMTP) transfers outgoing messages. Port 25 is commonly used between mail servers, port 587 for authenticated submission with STARTTLS, and port 465 for submission over implicit TLS. SPF, DKIM and DMARC affect message authentication and delivery policies rather than replacing SMTP transport.
Read SMTP response codes
Contain the full server response after EHLO, the authentication and the sending. 4xx. The most common is the temporary failure, while the 5xx. report a permanent refusal for this attempt. Detailed text and extended code are essential to distinguish quota, policy, authentication, relay or reputation. Check the same scenario after a single correction.
What is SMTP used for?
Send messages from email clients, websites, scanners and business applications.
Practical example
A website can submit contact-form messages to an authenticated SMTP server on port 587 using STARTTLS.
How can you test an SMTP server?
Check DNS resolution and TCP connectivity to the correct port for the service. For a server offering STARTTLS on port 587, use openssl s_client -starttls smtp -connect mail.example.com:587 to inspect TLS negotiation and the certificate. Then check the SMTP response codes and authentication policy.
Common problems
Common causes include blocked ports, incorrect credentials, disabled SMTP AUTH, rejected relay, TLS configuration errors, sending quotas and domain authentication or reputation policies. Read the full SMTP reply and enhanced status code to determine the actual cause.
Related terms
Key takeaway: SMTP handles outgoing email transport, while IMAP and POP3 are used to retrieve messages. Exact settings depend on the provider and application.
FAQ — SMTP….
What difference between ports 25 and 587?
Port 25 is mainly used for transfer between servers; 587 is commonly used for authenticated submission of clients or applications.
Does SMTP read emails?
No. Reading and synchronization use IMAP or POP3 instead; SMTP is used to send and transfer messages.
Does a SPF or DMARC error mean that the SMTP port is blocked?
No. Transport can work while a domain authentication or delivability policy then refuses the message.