SMTP Error 540 - Not a real code — usually 5.4.0, or a provider literal rejection. | Warmup Inbox
The short answer
Not a real code — usually 5.4.0, or a provider literal rejection. Common cause: Either enhanced status 5.4.0 (undefined network / routing) or a provider string such as Rambler Inactive / Blocked.
Quick fix
Read the bounce text; verify the recipient and your sender reputation.
5 - Permanent. The server will not try again. Fix the cause before you resend.
How to fix it
Read the bounce text; verify the recipient and your sender reputation.
Paired code
A real bounce often shows both a 3-digit code and an enhanced code together. This one commonly pairs with:
X.4.0 Other or undefined network / routing status
Related codes
511 Bad destination mailbox address — a misread of 5.1.1, not a real code. 513 Non-standard label: bad address syntax, or a relay/auth failure. 517 Non-standard label: bad sender attribute, or a policy block. 522 Mailbox full — a non-standard label for 552 / 5.2.2. 533 Non-standard label, usually a misread of 5.3.3. 510 Invalid recipient address — a vendor label, not a real code.
Ready when you are
Most rejections come back to sender reputation.
Blocklists, auth failures, and policy blocks are what warmup prevents. Warm up your domain so your mail is trusted before you hit send.