I'll chime in for Postmark as well.

As a B2B SaaS, our opportunity cost for support (and potential for lost goodwill) due to failed notifications is high, and switching to Postmark noticeably moved the needle. Time is our most valuable and scarce resource, and peer recommendation is our primary marketing device, so the price increment for switching to Postmark is easily justified. Their deliverability and support are stellar compared to Sendgrid, Mailgun, and SES.

Two thumbs up

See also: Fastmail over G suite for your mailboxes and calendaring.

In what way does your B2B use Postmark? And do you use Fastmail for your company mailboxes? If so, why not just use Postmark to forward emails?

Another vote for Postmark here. They are more expensive, but worth every penny.

I do wish they would let you choose the region where the server is located though, like Mailgun does.