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.
The "double hyphen" is a stand-in for an em dash (—), which is a punctuation usually used for expressing a pause before a related thought. Some of its functions are redundant with colons, semicolons, and even commas, although using it in place of a comma is typically frowned upon as unnecessary.
What you're dealing with in your greeting is an em dash taking the place of a colon or comma. The distinction is trivial, but to me it suggests a slight hesitation—the meaning of which could convey either an anxiety or something completely neutral.