Will the lesser known domain name make your emails more likely to be filtered as spam? I don’t know the answer, but I am fairly sure it wouldn’t help.
Will having your email routed through a middleman open up security issues? Probably solveable with diligence and awareness, but I recently had a non-technical friend with this setup get his Gmail breached because he was forwarding it to an email inbox on his personal domain from decades earlier that he forgot about, and didn’t have 2FA on the domain webmail. IMHO an easy oversight for anyone, honestly.
The domain name has no effect on spam filters, it’s all about which server is sending it and if the domain is properly configured with MX records, DKIM signing keys, etc… A custom domain with mail sent from a major mail provider won’t get sent to spam any more than a new gmail account.
Mailboxes for a custom domain work exactly the same as any other email. There is no forwarding to another mailbox or provider unless you configure it to be that way. You should be pretty careful with what settings the mail server is configured with though, because encryption is optional…
Two minor concerns about this approach:
Will the lesser known domain name make your emails more likely to be filtered as spam? I don’t know the answer, but I am fairly sure it wouldn’t help.
Will having your email routed through a middleman open up security issues? Probably solveable with diligence and awareness, but I recently had a non-technical friend with this setup get his Gmail breached because he was forwarding it to an email inbox on his personal domain from decades earlier that he forgot about, and didn’t have 2FA on the domain webmail. IMHO an easy oversight for anyone, honestly.