Throttle is a term used by hosting companies.
It limits the number of emails sent by site for a specified time, usually per hour..
This is just a security measure that the site is not spamming causing undue harm to other customers that may slow down the server. they are using.
Since this is a shared hosting plan there are many users using the same ip per server as us.
Because email is blacklisted by IP.
Then one customer could cause all other users on the same server to be banned or blacklisted.
