Problems sending e-mails -> unrouteable mail domain "hotmail.com"

hi,
i have some problems sending out e-mail with a small mailinglist (only 24 subscriber)
When sending the newsletter i becom error: unrouteable mail domain "domainname.com" This is fo every domain in the list (all 24 subscribers domains) I use this mailinglist several weeks and works fine. I also try to send mails via php and via SMTP but both give the same error.

Can anyone give me a tip what could be wrong?

thanks in advance

 

 

 

 

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