Thanks all for the responses.
Wondering if perhaps the Reset All Settings (on the iPad) will remove all the 'recents'?
Looks like we've been responding to the wrong question...
What you call "recents" (auto-fill suggestion of email addresses) is not a function of the mail account, or even the Mail app as such - it's a search function that's part of the operating system (if you use the Messages app, or FaceTime, you'd see similar behavior). It searches your Contacts. It may also contain a section called "Siri Found in Apps" ("Siri" in this case is a rebranding of Apple's in-device search functions, not the voice assistant), and yes, it may suggest addresses you have used or encountered recently but did not add to your Contacts.
Resetting All Settings will not "synchronize" this behavior - Reset All Settings (which is not available on Mac at all) does not affect data - it affects whether a particular feature is enabled/disabled by returning settings to their factory default. This could have impacts far beyond the issue you're trying to address, so unless you want to spend what could turn into a fair amount of time getting your iPhone or iPad back to the way you like things... don't do it.
Be more targeted - go into Settings > Siri & Search > Siri Suggestions (this is available on iOS, not in Mac System Preferences). If you want to learn more, tap the link in those settings, "About Siri Suggestions & Privacy..."
If you want these search results to synchronize, be sure Siri is enabled in the iCloud settings/preferences on all your devices.
While Apple continues to integrate the functioning of macOS and iOS, they are not identical - you will encounter settings/preferences and app functions in one that you will not be available on the other.
And as to getting a consistent Contacts search result across all your devices? Be sure your contacts are syncing in the cloud. Since you can have contacts stored in a variety of cloud accounts (nearly every mail service includes contacts), check to see that Contacts for each of those mail accounts is enabled/disabled consistently across all your devices, and that all mail accounts that contain contacts are setup on all your devices.