Currently, we throttle emails with a max send limit of 20 per minute per user.
We send emails as fast as possible. When we reach 20 emails sent in the current minute, the sending is paused until that minute expires, then another 20 sending jobs are queued. We do this is to minimize the chance of hitting the rate limit. Keep in mind that ALL operations on the user’s inbox, filing email activities, etc. count towards Google’s Rate limit. So if you're getting lots of activity on your account and we incrementally sync in response to each change, you could hit your rate limit without ever sending a single email. The number of emails sent is only part of what counts towards the rate limit.