TLDR: Make/use existing gmail account, setup a forwarder somehow for your custom domain, send email via the gmail smtp.
Dennis
1. Make a gmail account.
Just make a new gmail account or use your current one. Don’t use your custom domain here.
If you want a new account for your domain you can sign up for a new Gmail account at: https://accounts.google.com/signup/v2/webcreateaccount
2. Setup forwarder
If you use Namecheap or any domain registrar where you can add a forwarder to your domain, you can use this. Or you can use my favorite: https://improvmx.com/ to setup email forwarding.
NOTE: Don’t forget to set your MX DNS records!
3. Add 2 factor auth + make App password and setup Gmail so that you can use SMTP oh and don’t forget the SPF
Go to your Google account security settings and enable 2-Step verification.
Next make a new app password.
Once you have your app password. Add a Send mail as email address in the Accounts and Import tab in Gmail [click here for the lazy]
Use the mailserver smtp.gmail.com with port 465 – You sign in with your Gmail address and the newly created app password.
Don’t forget to set the SPF record for email deliverability 🙂
v=spf1 include:_spf.google.com ~all