Same exact boat, circa mid-2017. Was seeing lots of email go to spam on SendGrid, asked their support about it, and they suggested I purchase a monthly static IP. I moved to Postmark to save money and their deliverability ended up being WAY higher, due to the transaction-email-only policy.

But postmark is only transactional. Not mass mailing.

That’s the point, it helps keep the reputation of their servers high and ensures fast delivery of transactional email.

That might be why their deliverability is better :-)

+1 for postmark. Had deliverability issues with mailgun which were resolved once switching.

is deliverability purely based on the email server, or is it a combination between your domain/etc? I always wondered if it was YMMV for everyone saying different things about the different EMS out there (SendGrid, Mailgun, etc)

deliverability is actually incredibly complicated. Every mailbox provider (gmail, yahoo, hotmail/office365, comcast, etc... being the main ones) has their own algorithm for determining what is or isn't spam and includes things from user interaction to IP sending reputation to domain reputation to various types of authentication techniques (dmarc, dkim, spf, etc...)