OK, I think I've found the problem
Even with "Pop before SMTP" auth option checked, the printer doesn't authenticate when sending email. Capturing with wireshark this is the conversation
Printer
My server
- POP3
- USER impresora@xxxx.com
- OK Send your password
- PASS mypassinplaintext
- ACK
- QUIT
- HELO SMTP
- 250 Helo
- MAIL FROM: impresora@xxxx.com
- OK
- RCPT TO: paul@xxxx.com
- SMTP 530: SMTP authentication is required
- QUIT
No matter what I change on protocol settings the printer still tries to send without authentication