Mail.App, Rules, Regular Expression or content search, Base64
Mail and spotlight use the same process to construct and search the same metadata database. If you can’t search in spotlight, mail can’t search as well. 

To enhance the function of both, add a custom spotlight indexer to add that pattern to the indexing.

The default spotlight won’t find that content in spotlight in general or mail in specific.

If you’re of a mind to program your own extension here are two developer links to get started:


overview - https://developer.apple.com/design/human-interface-guidelines/macos/system-capabilities/search-and-spotlight/
details on the importer - https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/SpotlightCoreData/Articles/recordLevel.html#//apple_ref/doc/uid/TP40008065-CH101-SW1