The domain name doesn’t matter at all. The really important thing is the IP of your MTA (the server sending mails). Eg. if it was previously used to send spam you need to get a new one from your server provider, as it will be blocked everywhere. Also the server configuration needs to be good. Use dkim, dmark, spf, mta-sts, etc.

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I’m running the same setup, restic -> b2. Offsite I have a daily rclone job to pull (the diffs) from b2. Works perfectly, cost is < 1€ per month.