I've tried this. Most of my emails land in either "Promotions" or "Updates", or worse – Spam.
If you really want your emails to be delivered to most recipients Inbox – I would not recommend this.
To validate – please go to https://www.mail-tester.com – in 15 seconds it will evaluate and print your email score (you send it a dummy email). The main thing I see with using free gmail as a custom domain proxy SMTP is: It does not do DKIM (email signatures) except for @gmail.com or google workspace accounts. This reduces the sender score by a LOT.
Edit: The same is also true with forwardemail.net or any "incoming" email routing (free or paid). Because incoming email is just one step of using a custom domain. The main thing is the "Outgoing" email server. Unless outgoing email has full DKIM support for your custom domain, your emails are going to struggle reaching the recipient.
Post author here - I sent an email to mail-tester.com from an email alias I've set up as described in my post; it's gets a "Good stuff. Your email is almost perfect" and a score of 7.5/10
-0.5 from SpamAssassin (FREEMAIL_FORGED_FROMDOMAIN & HEADER_FROM_DIFFERENT_DOMAINS)
-1 for not being DKIM signed
-1 Because Google is in some blacklists
If you really want your emails to be delivered to most recipients Inbox – I would not recommend this.
To validate – please go to https://www.mail-tester.com – in 15 seconds it will evaluate and print your email score (you send it a dummy email). The main thing I see with using free gmail as a custom domain proxy SMTP is: It does not do DKIM (email signatures) except for @gmail.com or google workspace accounts. This reduces the sender score by a LOT.
Edit: The same is also true with forwardemail.net or any "incoming" email routing (free or paid). Because incoming email is just one step of using a custom domain. The main thing is the "Outgoing" email server. Unless outgoing email has full DKIM support for your custom domain, your emails are going to struggle reaching the recipient.