This only impacts Gmail web interface, according to [1]. You can use a desktop email client and continue to receive and send email from your custom domain.

"Web access changes: As previously announced, you can no longer add or sync your third-party email account in Gmail on the web. Alternative access: To access your third-party email account, you can use a dedicated mail app, a desktop mail client, or the Gmail mobile app."

[1] https://support.google.com/mail/answer/17101213?sjid=9352980...