cannot send to ten or more recipients through outlook.com

I am setting up a user who wishes to advise all contacts of a change of email address to outlook.com.
Using emclient and a new outlook.com 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 outlook.com

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

Regards,
Olivia