A: Hi,

It's Alex, the founder of ProxiedMail.
Yes, you're more than welcome to do it.
You have unlimited amount of custom domains with ProxiedMail for AppSumo Forever Exclusive plan.
It's actually a better way to do it. Since your custom domain is attached to the ProxiedMail, not to the email provider directly.
This way you can use your custom domain with multiple real emails - like cxplay@tuta.com and cxplay@gmail.com.

Also, the price-wise: usually you would need to pay for the custom domain on gmail or any other service 10 USD/month on average for one mailbox. Let's say you're using 3 of them. 30 per month just only for 1 domain. It's 360 USD annually.
With ProxiedMail you just pay 10 USD(thanks AppSumo for the discount) - and you go.
Unlimited amount of proxy emails and domains.

Isn't it's an amazing deal?

If you have any difficulty with setting up custom domain - text me here or to me email, since sometimes the DNS records are a bit confusing.