If you're thinking about switching from Gmail to FastMail, you can try out FastMail while still running everything through your Gmail account. This will allow you to try out FastMail without worrying about having any messages missing in Gmail if you decide you don't like it.
I am not official FastMail support, so proceed at your own risk.
- Forward all Gmail to Fastmail through the Gmail forwarding feature
- Set up a personality in FastMail that matches my Gmail address. It's in Settings > Advanced > Personalities in FastMail. The email address in this screen should match your Gmail address. Your external username is your full Gmail address.
- You can set your "Gmail" personality to the default if you want to always send mail from that account. You may need to go to Settings > Advanced > Account Preferences and scroll down to "From when replying" to set this on your desired setting to make sure the right personality is always used.
If you do this, you should receive all the mail sent to your Gmail in your FastMail account, and when you send outgoing mail it will be sent using Gmail's SMTP server so it will appear in your Gmail sent items.