I use Fastmail for personal e-mail for myself and two family members and where I work uses Microsoft's Office 365. I've never had issues with mails I send from Fastmail landing in spam, even to Google users, and I prefer their webmail UI over Outlook for the web or whatever it's called these days.

I just moved my domain mail to Fastmail, and everything arrives perfectly. I'm annoyed that I was bullied into this by the fact that 'unknown' mail servers are third-class citizens, but at least the problem is solved.

Some paranoid thread in me fears that Google will find a problem with Fastmail's IP range and credentials this spring (among others), in order to consolidate the Gmail upgrade cash avalanche, but I'm hoping that's just me with my tinfoil hat on too tight.

I've been a Fastmail customer for years and have never had an issue with being flagged.