5.1.3Low
smtp;501 5.1.3 Invalid address [outlook.com]
Outlook rejected the address as invalid. Fix the syntax, then resend. Remove the address if it stays malformed.
SMTP 501
Permanent (5xx)
The transaction failed permanently. The server will not try to send the message again. This is most commonly an issue with an incorrect/invalid email address.
Email providers
Outlook
1 responses for SMTP 501
Yahoo Inc
1 responses for SMTP 501
5.0.0Info
smtp;501 Syntax error in parameters or arguments
Yahoo rejected the SMTP command for bad syntax. Fix envelope or header formatting, then retry.
GMX
1 responses for SMTP 501
1Info
smtp;501 Syntax error in parameters or arguments
GMX refused reception due to incorrect server configuration. Fix the sending setup per their guidance, then request unblocking if needed.
1 responses for SMTP 501
1Info
smtp;501 Bad address syntax. http://service.exmail.qq.com/cgi-bin/help?subtype=1&&id=20022&&no=1000730
QQ rejected the address for bad syntax. Fix the envelope or header formatting, then retry.
Orange
1 responses for SMTP 501
1Info
smtp;501 5.1.0 Emetteur invalide. Invalid Sender. OFR004_405 [405]
Orange rejected the sender, usually a Return-Path or SPF alignment issue. Align the Return-Path with your sending domain and resend.
Response catalog adapted from the MIT-licensed SMTP Field Manual by ActiveCampaign / Postmark.