https://support.postbox-inc.com/hc/en-us/articles/218574818-5-7-1-END-OF-MESSAGE-End-of-data-rejected-Error

What are your smtp settings in Account Settings, Outgoing Server (SMTP), bottom of left pane?

"SSL/TLS", is that what you mean?

Reply here with this info:

outgoing server name, port, connection security, authentication method, User Name format, e.g. [email protected] or just user.

All right: outgoing server name: mail.hover.com Port: 465 Connection security: SSL/TLS authentification method: Password, normal User name format:[email protected]

Those are the correct settings according to the hover website, so I can't see any obvious reason why sending fails. AUP refers to Acceptable Use Policy, so I would contact hover to find out if you're account is being blocked in error.