What does it mean by "POPRELAY limit"

[a24uall]

I see the below in lfd logs.
Exceeded POPRELAY limit* from xxx.xxx.xxx.xxx (102 in the last hour)
what dies "POPRELAY" actually mean ?

csf.conf shows this value as 100

[a24uall]

Can someone explain this in simple terms?

[hostmart]

One of you users has sent a email with over 100 cc or bcc email recipient addresses

[Sergio]

That means that someone has sent more than 100 emails, it doesn't matter if it was to single email addresses or with a bcc or cc, in a frame of 1 hour (3600 secs), all the emails sent sum a total over 100 allow.