Implement a better way for email rate limiting to double our current sending rate

Our mail sponsor asks us, to rate limit our email sending to 1.000 mails in 10 minutes. We only reach this when sending newsletter or if there are very high peaks. This is the positive thing of this.
Our current implementation only allows us to send not even half of this.

Details can be read in our developer@foodsharing.network inbox:

"Re: Mögliche Email rate limits besser verstehen – Kunde 23955" (24.03.2020)

Current rate limiting in postfix only allows us to theoretically 60 mails per second which turns out to only be ~47 in reality as you can see here. The delay between mails of this limit only can be set higher :/

With !1419 (merged) there is a rate limit in php active which might not be the best solution. I implemented it, as I didn't get much help. It can make sense to keep it as Postfix seems to not send mails in first-in first-out manner when there is a large queue over 10k or 20k. Currently there is a low limit active.
!1419 (diffs)

A dragondreaming implementation of this would be a prioritisation system which allows giving newsletter and infomail stuff a very low prio while chat, store, foodsharepoint and forum mails have a higher prio.