large Mails from IPhone

He, I'm working as Mail administrators in the company, we assigned iPhone and iPad for each manager, two days ago i got a call from one of the managers complains that he is unable to forward one email from either the iPhone or the iPad, the email size is 11 MB and he is getting the following error : Cannot Send Mail, The message was rejected by the server, but if he tries to send the mail from the laptop or the Android device, it's working fine 😟

i tried to check for this error, but i couldn't find any solutions, just only on one forum, saying that; this is an IOS issue!!!!

for that kindly; i need to have your support for this issue please.