May be im wrong but why not set LIMIT of only X no. of mails can be Sent/Minute via user account. Find out how many emails people usually send per minute/hour

May be im wrong but why not set LIMIT of only X no. of mails can be Sent/Minute via user account. Find out how many emails people usually send per minute/hour and just DENY relaying anything else over that limit. That way it'll be less profitable for spammers to acquire user account details if he/she can only sent X mails every minute.