Hi,
The debug option did not deliver any information.....
About the mail settings:
In the general settings, the sending method is set to 'default PHP', not SMTP.
In the account settings, I can not set this options, so I assume, the account inherits this option from the general settings.
Still mailing does not work on my site. In addition to the above: the sent mail appears in the 'sent' folder (but is not received by the recipient).
What could migrating from version 1.5 to 1.5.1 change in my problem?