cannot send to ten or more recipients through

I am setting up a user who wishes to advise all contacts of a change of email address to
Using emclient and a new account, emails with a few recipients can be sent. A large number of recipients will not send, have reduced the number to ten and still will not send. How can this apparent anti-spam issue be overcome? The new account has been verified and security settings processed at

does this problem only happen in eM Client or does the same thing happen when they try to send a message directly from
Is there any error message/warning? Can you copy it for me if so?