Can't send emails but can receive them?

Can't send emails but can receive them?

So I'm not sure what's going on here, but I've been trying to send a copy of my pay slip to my account's email address and it just isn't arriving. He sent me a copy of his email address to me and that came through, but every time I try to send an email to him he never gets it. 

Anyone know what's going on here?