Add a daily SendGrid delivery rate option

Hello,

SendGrid has a free plan, but it is limited to 100 email messages delivery for each one day.

When one selects at your plugin to use SendGrid as the email delivery method – there is a feature to select from a fixed list of delivery rate options, but non of them is daily.

It will be great if you will add an option to send X (I guess 100 or less) emails per *day* (best if the number can be set manually by the user), as an upper limit and if the number of subscribers to the relevant delivery is above this number – the plugin will split the full number into the number of needed “chunks” of delivery for each day, until the list is done.

This way we will be able to use the free plan of SendGrid if we have more than 100 subscribers and we don’t care to send 100 emails or less emails per day.

Thank you.