Hey Oscar.
I am having the same problem with two of my hosting plans. That error means the server wont rely to multiple recipients at the same time.
I suggest you create a mail list within your CP and inside that list the recipients you have to send mail to.
Change the code of your util.php to send the e-mail to this mailing list instead of sending to sepparate recipients and see what happens. Maybe it works.
What you say is true, I have had several problems in the last few days and the support takes too long to answer. They should have backups though.
I hope I could help.
Best regards,