Hi Saroj,
First ask your service provider how many mails they provide and Is it hourly based or per day base, so suppose if you have hourly based lets say 500 Emails/Hour means after an hour you can send mails again.
And If you have a good knowledge of programming then please configure 2 accounts on SMTP class or make it function from different domain so that If one has reached the limit then use second function of second account.
Thanks