Problems mixing Exchange and Pop3 Email

All of our email addresses end are ______@totallifecounseling.com

1 Exchange email address is the master calendar _______@totallifecounseling.com

8 Pop3 email address are for therapists ________@totallifecounseling.com

Whenever emailing from the Exchange address to a Pop3 address with an appointment it often makes it to the Pop3 Inbox, but often we receive 4-8 “non deliverables and multiple “Delays”

I’ve been advised it will not work unless I convert all Pop3 to Exchange?

Is this true or is there a way to fix this?