if you're having this issue I would reach out to Fastmail support. A real human will dig through the smtp logs and find out what the issue is. Based on my experience... Fastmail is very on top of spam, blacklists and other deliverability issues.

Oh hey, I'm not alone!

In my case, it costed me two offers. One from Google and one from GitHub. Come to think of it, yeah I will begin migrating everything away from FastMail and stop using it completely.

One event that pissed me off with them: Instead of forwarding an email to SPAM, they simply throw it away instead. Turns out the email was not SPAM (2FA) and I had to go through weeks of support to find out what happened (thank god that you get a human for support but still).

Also I think they are still based in Australia? Would be happy to hear about alternatives.

I really really want to like Fastmail too, but it’s also cost me a job interview when the RSVP didn’t go through.

Are your domains set up correctly? That sounds suspiciously like something isn't set up correctly and their client's thought it was spam.

I use an @fastmail domain. The issue was how they handle RSVPs when your default calendar is an external one. RSVPing from within Fastmail attempted to Accept as my old Gmail address, who wasn't invited to the interview. On the interviewers end it looked like I didn't respond.

In some contexts, fastmail is viewed as a "throwaway" email provider. I ran into this when I was trying to provision a couple of VMs at a hosting provider. My fastmail email address was rejected. After contacting their support, they said they did not accept "throwaway" email addresses. I had to use a gmail acccount. The irony did not seem apparent to them.

My only scare with not using gmail and outlook calendars. However I moved to Zoho for mails and nextcloud self hosted for calendars. For mail delivery I make sure to verify again n again my SPF DKIM is valid or not but it’s a scare still :(

Isn’t it so bad that it’s overly hard to move away :(

With your own domain? Make sure you’ve got an the SPF, DKIM set up correctly, and register your domain with gmail’s postmaster tools. I moved my domain recently, and had completely forgotten about all that stuff I’d set up years ago. Started losing mails, redid the setup

Any references about spf, dkim, Gmail postmaster ?

I wonder if you can provide more context. I've only had issues when I tried to use external domains, e.g. gmail, as my reply-to when sending in fastmail.