CommuniMail - sendmail error after filling in signup form

[Silone]

Hi Support Team & Programmers,

after filling in my subscription form i always get the following error:

Software error:

error closing /usr/sbin/sendmail: (exit 768)

For help, please send mail to the webmaster (webmaster@***), giving this error message and the time and date of the error.

I thought this might be a configuration error, but i can't find a prob.
sending mails to hand-inserted-receivers works perfectly.

any idea to help me with`?
thanks