Is there a limit on how much I can send?

There is an outbound limit and your administrator can see it in their console, next to how much has been used.

It exists because everyone sending from a shared pool of addresses shares its reputation: one compromised account sending bulk mail would hurt delivery for every company alongside it.

What we do not do is advertise "unlimited" and let you discover a ceiling after you have moved in. If you need more, the number is a number and it can be changed.

Importing mail is never limited by it.