Not the OP but I wouldn't be surprised if they use both for different purposes. Postmark has extremely high delivery rates but is very particular about primarily sending transactional email like account updates, receipts, etc. They did recently add bulk API messages but it seems like a higher tier feature.
Mailgun is much more flexible about what they allow to go through their servers, but they have correspondingly lower delivery rates.
Mailgun is much more flexible about what they allow to go through their servers, but they have correspondingly lower delivery rates.