Thanks so much for the reply and links! I may have gotten it to work, thanks to the official guide you posted.
I uninstalled sendmail and replaced it with postfix.
Then I ran:
./bin/cake EmailQueue.sender
Passbolt sent off about 250 emails in the queue 
My email account instantly exceeded the maximum amount of emails per hour and discarded the rest. I DID received about 50 of them before this happened, so hopefully I can check again in an hour to see if it’s fixed.