Problem with mail on iphone

iOS and iPadOS

Hello to evrry one.
In the last time i bump in strange on the mail app on ios.
It's happend wen iam started too use on ios8.
Wen i try send a message to a specific group, i get the errow message billo:"Delivery to the following recipient failed permanently:".
It's happend onlly on my iphone.
I make shur that address is correct.
I know what what to do with this problem.