Hello RKNYC,
You may find the option to turn that off within Settings > Siri & Search > Mail. In there, you may turn off the options here, such as 'Learn from this App'.
This page can help out further with Siri Suggestions: Siri Suggestions on iPhone
Turn Siri Suggestions on or off for an app
Siri Suggestions are on by default for your apps. You can turn them off or change the settings at any time.
1. Go to Settings
> Siri & Search, scroll down, then select an app.
2. Turn settings on or off.
Change where Siri Suggestions appear
•Go to Settings
> Siri & Search, then turn on or off any of the following:
•Allow Notifications
•Show in App Library & Spotlight
•Show When Sharing
•Show When Listening
Cheers.