Sending mail from cms error

Hi,

I work with a small copany administrating their website and e-maisl. Somebody designed for us a CMS so we can for example easily send e-mails to people from our database. It was working ok, until now. It's supposed to be very simple: I have 4 fields to fill in:
1.) e-mail address
2.) title of the mail
3.) message in html
4.) message in text format.
When I do this now, as I always did bfore, instead of message that the e-mail sent ok, I get:
"There was a mail error: Language string failed to load: data_not_accepted

The only difference from thhe last time I sent a message is that the web space provider apparently upgraded their server and reupload our web files.

In the meantime I have change a password to our cms, but I can change all other files in cms, including adding members to database, so I didn't think it would affect anything. Anyway now I changed it back to what it was before and the mail still doesn't work.

I though maybe it was content of my mail, but it happens even when I try to send blank e-mail.

I know html, css, but not really the php (just now how it looks :)). I looked into our files and I can only see that when sending e-mail it supposed to link to smtp. As the server where upgraded could it affect the connections?

I need to send about 00 mails urgently, I hope somebody will have an idea.

I contacted web host support, but they are not coming back to me.

Thank you,

justeena