I have came across this issue quite a lot recently , where an external user is unable to send a mail to a specific users mailbox inside your exchange environment.
The first thing i did was check the anti-spam agents on the Edge Server and there was nothing denying a specific external user in any of the filters and as it was only one specific user inside the whole internal organisation who couldn’t receive mail from the external sender i turned my intentions elsewhere.
I then explored the avenue of Safelist aggregation. Safelist aggregation basically gets information out of a users Mailbox of the Safe Recipients Lists, Safe Senders Lists, Blocked Senders Lists and contact data and passes this information onto the anti-spam agents on the edge server.
The user then checked there blocked senders in outlook to find the external users email address stored in there, so they just took this out and the “554 5.1.0 Sender denied” NDR for the external user disappeared and the user received the mail 
Thanks for the post, Daniel! It got me in the right direction for fixing this issue. We did what you suggested (removing the email address from the block list and adding them to the safe senders list), but that didn’t quite fix it for us. I still had to go in and force Exchange to update the safe senders list for my user. The EMS command for this is:
Update-Safelist -Identity -Type SafeSenders
That fixed it for me!
Thank you!!!
Saved me lots of time…Thank you!