I’m having same issue. 
invoices are bouncing back with Error 3106 un able to send invoice code. 
I’ve double checked my clients email with him personally. Then I’ve sent him an actual message to his email from my personal email and his email received mail with no issues. 
I have 75 clients. Having no problems with there emails. Just his. 
error 3106 can’t send to recipient or un able to send check email message every time.