Why do emails get rejected / bounce back? PLESE HELP!

Hi!
One of my clients often send Emails to multiple recipients and she often gets Emails returned with an Mail Delivery Error.
Mostly the error says: "mailbox full" / "quota exceeded" OR "exceeded storage allocation" (552 Error)
It is very unlikely the recipients mailbox is full.
Sometimes the error says: "550 Data denied. (in reply to end of DATA command)" / "550 : Reason: mechanism (in reply to RCPT TO command)"

And also "535 authentication failure" which I believe could be the true reason for all of this ...

My client is really pissed and I don't know what it is.
I've got DKIM, SPF, SSL Authentification (Port 465, not TLS 587); Spamassasin, ClamAV, … up and running.

WHAT CAN IT BE?
PLEASE HELP!
THANK YOU

check your package limits email per hour... all this errors or most of them are remote errors mailbox is really full or maybe spam filters check is your ip blacklisted..

https://www.penpublishing.com/support/emailerrors/understandingemailerrors/