Three issues:
1) Sending or replay to any gmail address
Quote:
If you use an email service provider, verify that they authenticate your domain’s email with SPF and DKIM.
We recommend you always use the same domain for email authentication and hosting your public website.
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For instance all my hosted domains already have DKIM, but I also need to manually add SPF to my accounts on the hosting site.
https://support.google.com/a/answer/...401349025&rd=1
2) Using a desktop client (or real client on iOS or Android) to fetch email from gmail (POP3 or IMAP)
You need OAuth. You can't use different credentials. All you gmail address for one client must be connected.
https://support.mozilla.org/en-US/kb...bird-and-gmail
This applies to ANY desktop or real email client (K9 on Andriod). The Android Mail application is really giving Google the user accounts and passwords. Their servers then fetch and send that email, so I use K9 mail on Android.
3) Sending email from a desktop client, or real client on iOS or Android (SMTP).
I used to use one account and simply edit the from field. I can still do that with non-Gmail if I use my ISP SMTP. But if I use my hosting company SMTP, since last month I have to use a matching account. Not a problem. My desktop client can have as many accounts as I need.
If the email from address is a gmail account I can no longer since some years ago use the ISP. You have to use the google SMTP. For this on an email client you need OAuth.
This is actually all more about Google being able to scan all emails to and from all gmail accounts for their 'slurping'. It's delusional to believe that phishing and spammers will be stopped.
I receive all gmail by having forwarding to non-gmail accounts on all my gmail accounts since OAuth came in a few years ago. Till last month I simply spoofed the gmail 'from' via my ISP if I had to send/reply from a gmail account and had no problem sending to any gmail user via my hosting company SMTP or ISP from my own domains' email addresses.
But now I can't send any email to a gmail recipent from my own domain email addresses till I figure out SPF. The Website of my Hosting company is entirely in German.
Meanwhile I have to use Gmail on web or Android to Gmail users and can only use my Gmail addresses, not my "proper ones".
Another issue is that some ISPs ONLY let you use their SMTP, they block the ports to all others. Unless they have SPF and DKIM you can't send to a gmail account!
But really Google want you to use their Gmail on Web, ChromeOS and Android for gmail from addresses (or their SMTP), because they want to 'read' not just all incoming email to gmail addresses from anyone (which they have always done), but also read all outgoing email from any gmail address. Also they want you to to use Gmail, not your ISP email nor your paid for domain hosting company email.
Gmail is free because the content and metadata is useful to Google.
Whitelisting won't work.