I usually see just format in the emails I receive daily:
I'm calling you in regard with the something...
I see comma after the greetings too but I am not sure why double hyphen is being used there. Can someone explain it?
Haha, I don't know what their deal is. After a salutation, use a comma for a personal letter, a colon for a business letter, and either for an email. Dashes are not acceptable.
Dear Mr. Lawrence:
Dear Kane (,/:)
I don't know why someone would put a double hyphen after the recipient's name in an email, but the Internet has long-standing tradition of the "sigdash". In early email and news software, the convention was adopted that anything below a line containing two dashes and a space, was your signature block.
Most email and news software to this day respects this de facto convention, although, to my knowledge, it never made it into any Internet standard or RFC.