An email client needs to know the IP address of its initial SMTP server and this has to be given as part of its configuration (usually given as a DNS name). This server will deliver outgoing messages on behalf of the user.
Outgoing mail server access restrictions
Server administrators need to impose some control on which clients can use the server. This enables them to deal with abuse, for example spam. Two solutions have been in common use:
- In the past, many systems imposed usage restrictions by the location of the client, only permitting usage by clients whose IP address is one that the server administrators control. Usage from any other client IP address is disallowed.
- Modern SMTP servers typically offer an alternative system that requires authentication of clients by credentials before allowing access.
Restricting access by location
Under this system, an ISP’s SMTP server will not allow access by users who are outside the ISP’s network. More precisely, the server may only allow access to users with an IP address provided by the ISP, which is equivalent to requiring that they are connected to the Internet using that same ISP.
This system has several variations. For example, an organisation’s SMTP server may only provide service to users on the same network, enforcing this by firewalling to block access by users on the wider Internet. Or the server may perform range checks on the client’s IP address.
Client authentication
Modern SMTP servers typically require authentication of clients by credentials before allowing access, rather than restricting access by location as described earlier. This more flexible system is friendly to mobile users and allows them to have a fixed choice of configured outbound SMTP server.
Open relay
A server that is accessible on the wider Internet and does not enforce these kinds of access restrictions is known as an open relay. This is now generally considered a bad practice worthy of blacklisting.
Ports
Communication between mail servers generally uses the standard TCP port 25 designated for SMTP.
Mail clients however generally don’t use this, instead using specific “submission” ports. Mail services generally accept email submission from clients on one of:
- 587 (Submission), as formalized in RFC 6409 (previously RFC 2476)
- 465 This port was deprecated after RFC 2487, until the issue of RFC 8314.
Port 2525 and others may be used by some individual providers, but have never been officially supported.
Most Internet service providers now block all outgoing port 25 traffic from their customers as an anti-spam measure. For the same reason, businesses will typically configure their firewall to only allow outgoing port 25 traffic from their designated mail servers.
SMTP transport example
A typical example of sending a message via SMTP to two mailboxes (alice and theboss) located in the same mail domain (example.com or localhost.com) is reproduced in the following session exchange. (In this example, the conversation parts are prefixed with S: and C:, for server and client, respectively; these labels are not part of the exchange.)
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