Email provider · 5.x.x permanent failure
Every SMTP 501 response on record from Yahoo Inc (yahoo.com, aol.com, verizon.net), with the verbatim text, the enhanced status code, a severity rating and a plain-English fix.
A syntax error in the command arguments, almost always a malformed or invalid address in the envelope or headers. It stays permanent until the address is corrected.
smtp;501 Syntax error in parameters or arguments
The receiver refused this message outright and will not accept an unchanged retry. The wording above is the only clue to what needs to change, most often the address, authentication, or sending reputation.
Provider documentation
Postmaster & documentation
A syntax error in the command arguments, almost always a malformed or invalid address in the envelope or headers. It stays permanent until the address is corrected. At Yahoo Inc, it is a permanent rejection, and the specific reason is in the enhanced status code and message text of each response below.
Match your bounce text to one of the responses below and follow its guidance. Recipient-side wording means fix the address. Authentication or reputation wording (SPF, DKIM, DMARC, blocklist) means fix your sending setup, and check Yahoo Inc's postmaster documentation.
