How can email blocking be implemented?

Can emails be blocked by subject line? That is, can a private or
commerical individual user of email choose to block emails that contain
a particular Subject?

Would that block it from everyone using the same domain?

It says "550 permanent"

This is the relevant part of the bounce message:

<jo-----@d--------.com>: host
d142803a.ess.barracudanetworks.com[209.222.82.252] said: 550
permanent
failure for one or more recipients
(m---@d--------.com:blocked,j------@d--------.com:blocked)
(in reply to end of DATA command)

<me----@d----------.com>: host
d142803a.ess.barracudanetworks.com[209.222.82.252] said: 550
permanent
failure for one or more recipients
(m-----@d-----------.com:blocked,j------@d---------.com:blocked)
(in reply to end of DATA command)

As an aside, is it proper for an employee P to block emails to her boss?
This is a case where it took more than 47 days and she only got one
proposal for removal of a tree that had fallen on the back yard of one
of the homes in the HOA she is managing agent for. And that company
wanted to charge $2300 when another licensed, insured company later did
the work and did a great job start to finish for $650. Was P going to
get a kickback from the first company and is that why she doesn't want
her boss reading more about it, or is it reasonable to just keep him
from getting too much email?