Control Panel V5 > Account Center > Add ons
As an ASP.NET hosting company, we fully commit all of our resources to do ASP.NET hosting. Basic email service is a free service that we offer to people who have low email usages. Emails that can be sent per hour per domain of all shared plan packages are 50 by default. 
Sent-Limit100 Additional 100 emails per hour(Using WebMail or Email Client like outlook), this product is applied to the whole domain email service. With it, you can send additional 100 email messages per hour per domain. Please note that this add on applied to domain. One add on for one domain name and you can order 2 or more sent-limit 100 for one domain.
How to order it?

Go to member account ---> Add ons, We do support quarterly or yearly payment on this add-on.

When you click to buy, the system will let you choose which hosting plan and which domain you want to apply this add-on.
How to use it?
After you order this add-on, the quota will be auto-applied on your domain.  For example, you order one sent limit 100 add-on, then your domain hourly sent limit will be increased to 150.