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
howpublished; and I'm personally quite fond of the
note field for adding sundries such as the job title of this personal communicator, and the name of his or her employer.
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.
[Edited January 2017 to add]
If somebody wants to add a working example to this answer, they are more than welcome. It has been years since I've used either LaTeX or BibTeX, so it would take me some time to figure out myself. In the meantime, here is an answer by Marco Daniel on another question contains code + result of an
@misc entry that uses the