SMTP Protocol Error: 501 5.1.7 Bad sender address syntax when sending mail from our printer through Nethserver - Support - NethServer Community
Everything you need to know about SMTP Error Codes | Pepipost
501 5.1.7 Bad sender address syntax - Technical assistance - Zammad - Community
Everything you need to know about SMTP Error Codes | Pepipost
Bad recipient address syntax when sending to an email ist | Thunderbird Support Forum | Mozilla Support
With email in outbox: Failed to send some messages. Negative SMTP reply 501 5.1.3 Bad recipient address syntax · Issue #1328 · thunderbird/thunderbird-android · GitHub