SMTP error 504 5.7.4: unrecognized authentication type

SMTP error 504 5.7.4 means the server does not support the specific authentication mechanism

your client requested. The enhanced status code 5.7.4 is literally defined as "security

features not supported" - the server has authentication available, just not the particular

method (CRAM-MD5, NTLM, GSSAPI, a deprecated LOGIN variant) your client tried to use.

Fault: sender configuration. This has nothing to do with your username or password being

correct - the server never got far enough to check them.

Why it happens

• A mail library defaults to an older or stronger mechanism the server dropped. Some SMTP

clients default to CRAM-MD5 for perceived security, but many modern hosted mail providers

have removed it in favour of LOGIN/PLAIN over TLS, or OAuth-based XOAUTH2.

• A mismatch between the advertised and requested mechanism. The server's EHLO response

lists exactly which AUTH mechanisms it supports on that connection (which can differ between

port 587 and 465, and before/after STARTTLS). A client that ignores this list and always sends

the same AUTH verb will eventually hit a server that does not offer it.

• Legacy relay software that only understands one mechanism talking to a modern server that

requires a different one, often surfaced after a provider deprecates basic LOGIN auth in

favour of OAuth.

• Connecting on the wrong port for the intended mechanism - some providers only offer certain

auth types pre-STARTTLS versus post-STARTTLS.

• A library configuration that lists mechanisms in a fixed preference order without checking

whether the first-preferred one is actually available on this specific connection, so it tries

and fails on the unsupported mechanism before ever attempting one that would have worked.

None of these causes are intermittent in the way a reputation-based failure would be. If a given

client and server combination produces 504 5.7.4 once, it will produce it every time under the

same configuration, which is itself a useful diagnostic signal: a consistently reproducible

failure points at a mechanism mismatch, not at anything transient on either end.

The transcript

$ swaks --to test@mailpilots.in --server smtp.example.net:587 \
        --auth CRAM-MD5 --auth-user sender@yourdomain.com -tls
-> EHLO sender.example.com
<- 250-smtp.example.net Hello
<- 250-AUTH LOGIN PLAIN XOAUTH2
<- 250 STARTTLS
-> AUTH CRAM-MD5
<- 504 5.7.4 Unrecognized authentication type

The EHLO response already told you the answer: AUTH LOGIN PLAIN XOAUTH2 was offered, and

CRAM-MD5 was not in that list. The server's complaint is accurate.

How the big three word it

Diagnose it

$ swaks --to test@mailpilots.in --server smtp.example.net:587 -tls --quit-after EHLO
-> EHLO sender.example.com
<- 250-smtp.example.net Hello
<- 250-AUTH LOGIN PLAIN XOAUTH2
<- 250 STARTTLS

--quit-after EHLO shows the advertised AUTH line without attempting a login at all - the

fastest way to confirm which mechanisms are actually on offer before changing any client

configuration.

The fix, in order

1. Read the `EHLO` response first, every time, rather than guessing a mechanism.
2. Switch your client's configured mechanism to one in the advertised list - almost always

LOGIN or PLAIN over TLS, or OAuth where the provider offers it.

1. Confirm you are connecting over TLS before attempting `AUTH` at all. Several providers

only advertise authentication mechanisms after STARTTLS succeeds; a plaintext connection may

show a shorter AUTH line or none at all.

1. Update the mail library if it hard-codes a mechanism with no fallback - most modern

libraries negotiate automatically from the EHLO response and only need the right credential

type configured.

1. Move to OAuth where available, which sidesteps mechanism negotiation entirely for Gmail

and Outlook.

What not to do

• Do not change the password. The mechanism, not the credential, is the problem.
• Do not assume the account is misconfigured on the server side. The server told you exactly

what it supports in the same transcript that produced the error.

• Do not fall back to disabling TLS to try a different mechanism in plaintext - several

providers refuse authentication outright on an unencrypted connection, which produces a

different error, not a fix.

• Do not hard-code a single mechanism in custom scripts without first checking the EHLO

response at connection time, since a provider can change its supported list without notice.

MailPilot's SMTP/IMAP connections negotiate authentication automatically against whatever the

provider's EHLO response advertises, and its OAuth-based Gmail and Outlook connections remove

mechanism negotiation from the picture entirely.