What's different between "250-AUTH LOGIN PLAIN" and "250-AUTH=LOGIN PLAIN"?

Question

I can see many ehlo command echo from mail server like this:

I want to know what's different between "250-AUTH LOGIN PLAIN" and "250-AUTH=LOGIN PLAIN"? Thanks

Answer 1

They mean the same thing, but one is for backwards compatibility.

The first one, 250-AUTH LOGIN PLAIN, is the modern RFC-compliant way for a mail server to announce what authentication methods it supports. Authenticated SMTP is used to allow valid remote users to send mail through an SMTP server while still preventing spammers from connecting and using it as a relay.

The second example, 250-AUTH=LOGIN PLAIN, is an identical command but is intended for non-RFC-compliant email clients. Older clients, such as Outlook 2003 and older, did not implement the AUTH command correctly and instead expected an equals sign between AUTH and the available authentication methods.

For backwards compatibility, it is possible to enable both the compliant and non-compliant AUTH commands. In Postfix, for example, you can enable the broken_sasl_auth_clients option to force Postfix to broadcast AUTH twice. Otherwise, only the RFC-compliant AUTH command should be used.