Which bibtex entry would should I use to cite a personal correspondence via email. Also, If I want to add the function and company (say financial controller at pencils inc.) of the person cited, how to do it?
There's a list of the fields vanilla BibTeX uses on page 13 of Tame the BeaST (ttb-en.pdf). I'd stick "Personal communication" in
I do not know what citation style your style guide requires, but don't be afraid to deviate from it if it did not foresee this situation. The world will so not end.