Emails to AOL address problem

My Mail version is 11.5 and my iMac v 10.13..6 High Sierra. My friend uses an iPhone 11 on iOS 13.3.
I have tried sending him emails from my iPad but with the same negative result. My emails to other addresses appear to be be received. I haven’t yet identified the date when friend stopped seeing my emails. Am trying to see if the problem occurred when he upgraded to an iPhone 11.