Usually it means the system that returned the notice to you has shutdown
the mail service for the particular mailbox listed - because the human
(their service subscriber) affiliated with the mailbox has not bothered
to visit their site to look at the mail, for some period of time defined
by the service administrator.

In other words, the person affiliated with the "[log in to unmask]"
mailbox is not looking at the mail any more, so the service provider has
stopped accepting new mail into that box.  The phrase "extended
inactivity" refers to however long the administrator has chosen to wait,
since the last time the person logged in.  This period varies by
service, for some sites it is 30 days.  The person may have fallen ill,
or taken an extended vacation without internet access, or simply decided
that they just don't care about that particular mailbox.

>>> [log in to unmask] 6/26/2007 4:25:53 PM >>>
Hello to anyo9ne:

5.1.1 550: user@domain.com...Due  to extended inactivity new mail is
not currently being accepted for this mailbox.

I wonder what does that means? I really don't understand this.

Thanks!
Johnny :)