Thanks all!

6am, 6 am, 6 AM, 6 a.m., 6 A.M., 6 o'clock, 06:00, or six o'clock are all correct. Personally I use 06:00 or 6am usually. It's a stylistic choice.

The most grammatically correct is using "a.m." but in both business and casual written exchanges I've seen all of the above used.

The Chicago Manual of Style (CMS) is available online.

6 a.m. - It's six o'clock.