mail groups

A recent problem has developed in that I seem unable to send emails to Contact Group addresses from my iMac using Mac Mail. When I try, I receive a Mail Delivery Subsystem failure notification to the effect that the message has been rejected by the remote server. Previously I had no difficulty and am not aware that I have made any inadvertent changes to my settings. Any advice would be welcome. Also, is there any maximum to the number of addresses that can be included in a Group for emailing purposes?