How much are you paying for Postmark?
The cheapest plan on SparkPost starts at $20 per month for 50K emails, plus an additional $20 per month for a dedicated IP address = $40 pm or $0.80 per thousand emails
The cheapest plan on Postmark is $10 pm for only 10K emails, plus an additional $50 pm for dedicated IP = $60 pm or $6.00 per thousand emails.
For 50K emails on Postmark, you're looking at $100pm or $2.00 per thousand.
100K emails on Postmark is $150pm or $1.50 per thousand - while for $75 pm on SparkPost you can get 100K emails with a dedicated IP ... literally half the price.
(you really do want a dedicated IP in my experience - it only takes one bad sender on a shared IP to ruin your sending reputation).
I'm currently sending around 85K emails per month on my SparkPost account across my several dozen websites and paying $75pm for it.
So while there's no official plugin for SparkPost, I do have this free addon which provides full bounced email support and I have been running it in production on 6 of my 8 sites:
https://xenforo.com/community/resources/sparkpost-mail-transport-for-xf-2-2.7990/ (while the previous unreleased version of this addon for v1.5 is running on my other 2 sites).
Just giving you some alternatives to consider!