To start, email is not private as a concept itself. Your emails are stored in the recipient's computer/server when sent, so any measures are ineffective if your recipient lacks them as well. Objectively speaking, I have a pretty neutral stance on using Google and its services as they are very convenient and just work; however, I dislike relying on them because of the lack of control and privacy. Being forced into adding phone numbers for 2FA or having to deal with, in my opinion, terrible user interface was not for me. Gmail is also so dominant that it's unusual to hear anything else after the @ in email addresses. While this is anecdotal, it still results in spam filters treating Gmail accounts much better (true for other big players like Outlook as well) than other providers. In short, besides the privacy concerns, Google was just straight up annoying to deal with for me.
Furthermore, Google is also notorious for selling data to advertisers and logging everything to make your advertising profile better and more tailored to you. That's how they make money, and this is common knowledge. It's totally subjective, but I, for one, want to be in more control of my communication and accounts associated with my email. I don't want tailored ads. There are pros and cons to everything; you will probably have to give up features in exchange for more control. I've used a considerable amount of services, and I've always found at least one thing that makes me unhappy.
For now, I use ProtonMail and SimpleLogin with Cloudflare as my domain registrar to manage my emails and custom domains as addresses. It's a simple system but rather complicated in a way. I'm gonna explain it via the two domains that I use, lets call them mydomain.com and sub.mydomain.com for this post. As you can tell, sub.mydomain.com is a subdomain of mydomain.com.
My ProtonMail address is my main inbox, where all the emails go. It has end-to-end PGP encryption, at least while communicating with other ProtonMail accounts. Aliases are made that point to the main @protonmail.com address.
Email alias are simply addresses that forward all incoming emails to the address they're an alias of.
SimpleLogin takes care of sub.mydomain.com and has many aliases created for it. These aliases correspond to their own service, so my main address stays hidden, for example:
[email protected] is used exclusively for my amazon account.[email protected] is used exclusively for my reddit account.Each of these aliases points to my main inbox.
ProtonMail takes care of mydomain.com and has a few aliases created for it. These aliases are for my personal use for a nice custom address while communicating professionally or privately, for example:
[email protected] for private communication.[email protected] for public communication.Like the SimpleLogin aliases, these also funnel directly into my main inbox, but its easier to send emails from custom addresses from ProtonMail because I dont need to rely on reverse aliases to send emails.
This system has many benefits. I can control spam because each service has its own alias, simply speaking, its own email. So, if I ever receive too many emails from a service, I can just disable/delete the corresponding alias and go about my day. It also gives me peace of mind that my main email is private and hidden from these services that harvest tons of customer data for promotions, etc.
Using a custom domain also keeps moving between services very easy should ProtonMail shut down in the future. Although this is a pretty good system, it's only really to hide from spam and decentralize my regristrations because true email privacy is very hard to achieve as email as a concept is not private.