problems with sending to several recipients

[Zaphod Breeblebox]

Hi

I have problems with sending a message to several recipients; in the past
this worked out ; adress book --> to or cc or bcc and then select several
addresses and send the message

Now i get this;

could not send..... error type ' unroutable address' , bla bla bla.....

When i send the message to one or very few recipients there is no problem.

What is the cause and the solution to this problem?

Thx

Mick