Is a 554 always spam related?

No. It can also mean a relay attempt, a refused connection, or a content rule. The enhanced code and text tell you which.

What does a 554 right after connecting mean?

The server refused you before any mail was sent, which almost always points at your IP reputation or a blocklist.

SMTP 554: transaction failed

554Transaction failedPermanent: retrying will not help

Example

554 5.7.1 <user@example.com>: Relay access denied

What it means

RFC 5321 defines 554 as "Transaction failed", and adds that in reply to the connection opening it means "No SMTP service here". So it can appear in three places: as the greeting (the server will not talk to you at all), at RCPT TO, or after the message body has been sent.

Receivers mostly use 554 for policy and content decisions. 554 5.7.1 is common for spam verdicts and for relay attempts. Yahoo uses 554 5.7.9 for messages that fail its DMARC or authentication policy, Proofpoint uses 554 5.7.1 for content and reputation blocks, and Zoho uses 554 5.1.8 for rejected senders. A 554 in the greeting often means your IP is on a blocklist the receiver checks before it says hello.

It is permanent. 550 overlaps heavily; many systems choose 550 for recipient level refusals and 554 for whole transaction or connection refusals.

Likely causes

The receiver scored the message as spam or found malware after DATA.
Your IP is on a DNSBL and the receiver refuses the connection with a 554 greeting.
You tried to relay through a server that does not accept relaying for you ("Relay access denied").
The message failed DMARC or other authentication policy (Yahoo 554 5.7.9).
An attachment type or size rule on the recipient’s side blocked the message.

How to fix it

Note at which stage the 554 came: greeting, RCPT TO, or after DATA. That tells you whether it is your IP, the address, or the content.
For greeting rejections, check your IP on blocklists and request removal once the cause is fixed.
For relay denials, authenticate on the submission port or send through your own domain’s server.
For content and authentication blocks, check SPF, DKIM, and DMARC alignment, and strip unusual attachments or link shorteners before resending.
If you are the recipient’s admin, check your gateway’s message trace for the rule that matched.

Checks that apply

Your domain or sending IP (fills in the checks below)

Blacklist Check Finds the DNSBL listings that commonly trigger a 554 at connection time.
DMARC Check Checks your DMARC policy and record syntax, important for Yahoo and other receivers that return 554 on DMARC failures.
DKIM Check Verifies your DKIM selector is published and the key parses, the piece most often missing when 554 cites authentication.
Google/Yahoo Compliance Tests your domain against the Google and Yahoo bulk sender rules that sit behind many 554 rejections.

FAQ

Is a 554 always spam related?: No. It can also mean a relay attempt, a refused connection, or a content rule. The enhanced code and text tell you which.
What does a 554 right after connecting mean?: The server refused you before any mail was sent, which almost always points at your IP reputation or a blocklist.

All 191 bounce codes