[imp] IMP does suddenly not work with sendmail

[BiBe]

Hi,
i'm using IMP 4.1.1 with same configuration for months and now 
from one day to the other, composed mails are not be sent. There
is no error displayed, it seems everthing working ok, but nothing
happens in the SMTP-log and the mail ist not sent.

I've compared (diff) the config of the working version from april,
nothing has changed.

Other php-Skripts can send Mails correctly using sendmail.

Does anybody has an idea, why IMP on my Mailserver is suddenly not
able to send mails?

Many thanks,
BiBe