SMTP Error (452):Failed to add recipient ... too many recipients in last hour

[#1]

I've just tried using webmail to reply to an important email regarding a money transfer and received the message:

SMTP Error (452):Failed to add recipient "[emailaddress@talktalk.net]" (<[emailaddress@talktalk.net]> too many recipients in last hour).
Googling that resulted in an explanation that exceeded my understanding (http://www.mimecast.com/mc/kb/Mimecast/KBID10473.htm) - though suggested it was a temporary glitch. I also found http://community.plus.net/forum/index.php?topic=74985.0 in the archive here via Google.
After trying to send three times - then saving as draft and having another failure when resent, I gave up and used my Gmail account instead.
Though I've now resolved that matter perhaps someone could explain what's wrong with Plusnet sending that email this evening - in simple language please?
XPSP3 FF16.0.2

[#2]

It's a problem this side. When we moved the Webmail platform last week, we neglected to add an exception to our outbound anti-spam appliances for the new IP's. This means they were hitting the 1000 message/hour rate limit that's in place. It's been fixed now so should no longer be a problem.

[#3]

Thank you