fix: properly require 1 element in `MailDataRequired` type

[shammahatdivvy]

Current type does not work with the latest versions of typescript. This updates the type so it remains an array, and not the intersection of an object and an array.

Fixes

A short description of what this PR does.

Checklist

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• [x] I have made a material change to the repo (functionality, testing, spelling, grammar)
• [x] I have read the Contribution Guidelines and my PR follows them
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• [x] I have added tests that prove my fix is effective or that my feature works
• [x] I have added the necessary documentation about the functionality in the appropriate .md file
• [x] I have added inline documentation to the code I modified

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[htwnavarro]

Would appreciate seeing this released 🙂

Thanks!