To combat threats such as dictionary and DoS attacks inbound mail is rate-limited in the following manner, after which automatic blacklisting occurs...

Sender Limits
Description	Period	Limit
Messages	24 hours	5,000
Recipients	24 hours	5,000
Traffic Quota	24 hours	250Mbyte

Recipient Limits
Description	Period	Limit
Messages	1 hour	64

If you're a bulk mail provider and you require our limit(s) to be raised, you may direct your request with the following information to our abuse department.

Company name

Contact details

Motivation for us increasing the limit(s)

Mailserver details: IP/Netblock, Hostname, HELO/EHLO string

Full description of what mail you're sending