fo you use the mass mail plugin 

I used the feature that comes with Oxwall

try reproduce it  by set ur self up and few other accounts (maybe more yours) as roll admin or something and send mass mail to admin roll so only you get it
the if your get the lots emails 
then send mass mail to your self again and monitor db base mail and look if the mail in there has the extra
if there is the extra in base mail db then maybe you changed the mail templates and stuffed them up 

if is not in the db base mail   then the problems your mail server  

Kim, due to following facts I believe that the issue is not connected with our software:
1. We've try to reproduce this issue on our test sites and everything work properly.
2. According to your post, you also could not reproduce this issue.
3. Not all users receive the message with an extra email addresses.
4. Why is it 22 email addresses? By what criterion or principle they were selected. Do they have something general?

Our assumption is that the letter was sent manually. To make sure, we need more information about the letter. Please PM me the origin letter with headers. To get this letter you will need go to the email service settings and find option "Show original". Save this file and PM me.