In 2026 Gmail removed the feature for fetching emails from third-party accounts with POP3.
Some of my clients liked using Gmail with a custom domain and were using this feature. Google were trying to convert users to their Workspace product which lets you use a custom domain, however it seems to be very fussy what domains will work with it. If it thinks your email is spam it seems there is nothing you can do about it. Domain verification doesn’t help. Support seem unable to help.
After extensive searching for a solution I found Gomailify which works with Googles official APIs. It took about 30 minutes to set up a client and required some changes to DNS and Gmail settings. It costs just $12 per year for 2 linked Gmail accounts.