What your library says, and what the server said

Almost no library shows you the refusal. It catches the server’s line, throws away everything except the fact that something failed, and prints a sentence of its own — the same sentence it has printed for every SMTP failure for a decade.

So the message you are searching for is not the message that explains anything. This table connects the two.

Your library prints	The server actually said
`SMTP Error: Could not authenticate.` PHP · PHPMailer	`Authentication unsuccessful, basic authentication is disabled`
`SMTP connect() failed.` PHP · WordPress / WP Mail SMTP	`Authentication unsuccessful, basic authentication is disabled`
`Invalid login: 535 5.7.139 Authentication unsuccessful` Node.js · Nodemailer	`Authentication unsuccessful, basic authentication is disabled`
`smtplib.SMTPAuthenticationError: (535, b'5.7.139 ...')` Python · Python smtplib	`Authentication unsuccessful, basic authentication is disabled`
`curl: (67) Login denied` shell · curl	`Authentication unsuccessful, basic authentication is disabled`
`SmtpException: The SMTP server requires a secure connection or the client was not authenticated.` .NET · .NET and PowerShell	`Authentication unsuccessful, basic authentication is disabled`

Why this is worth a table

Three of the four things that produce these errors are reversible by anybody with tenant rights. The fourth is the end-of-December-2026 default change, and an administrator can undo that one too — until Microsoft names the final removal date, which it says it will do in the second half of 2027.

The library’s message cannot tell them apart. The server’s line always can. Every page above starts with how to make your library print it.

Missing your library? Send the exact string and what produced it.

Last reviewed 2026-08-27.

Updated 29 Aug 2026

What your library says, and what the server said

Why this is worth a table

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