By default, Hostens servers allow to send messages via an unencrypted SMTP channel, but it’s limited by a factor of 10, eg. if the limit by default is 100, you’ll be able to send 10/mails hour (from each mail account).
In order to be able to send a larger amount of messages, you need to enable SSL encryption for the SMTP (outgoing) mail server. You can achieve it with only a few clicks in the preferences menu:
Change the SMTP server name to your server hostname (eg. example.hostens.cloud). The exact name can be found in your Client area;
Change SMTP port to 587;
Enable SMTP encryption.