Blocking of email

I have been sending emails to artists for 15 years now inviting them to attend workshops at my studio, presented by Professional Artists. All of a sudden MWeb is blocking my invitations. Surely the recipient can choose whether or not they want to receive the invitations. I give them all the opportunity to Unsubscribe if they no longer want to receive my mail. A number of recipients are now using alternate addresses What can I do to fix this problem.?