Sending email from Fisheye fails due to 501 Syntax: HELO hostname error | Fisheye | Atlassian Documentation
How to fix Error 501 (SMTP -> ERROR: MAIL not accepted from server: 501 : sender address) - 501 : sender address must contain a domain SMTP -> ERROR: MAIL not accepted from server: 501 : sender address.
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How to Fix the HTTP 501 Not Implemented Error
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How to Fix the HTTP 501 Not Implemented Error
SMTP server response: 501 5.1.7 Bad sender address syntax in... · Issue #169 · swiftmailer/swiftmailer · GitHub
501 5.1.7 Bad sender address syntax` in SMTP username with @domain.tld part · Issue #3181 · zammad/zammad · GitHub