What limits are there on hosted email plans?

We have a hard limit of 300 outbound messages an hour (and in turn 7200 messages a day) on all accounts, regardless of their size. The Starter plan is further limited to just 100 outbound messages per day, as it's intended for personal use, and that's a lot of email writing per day! If you reach this send quota, your outbound mail will be suspended for the remainder of that hour. This also triggers an alert, and it will be investigated. If you aren't regularly approaching this limit, you should be fine. However, if you're found to be sending what even remotely looks like unsolicited email, your account will be suspended immediately. We're serious here, spammers are not welcome! Note that we're not looking at the content of your email. There are certain signatures and bounce rates that make it very clear that mail is being sent unsolicited.

The only other limits are your storage quotas, which are on a per account basis, not per address or domain. Your quota is shared across all of your domains and email addresses.