Dear Sir/Madam or Dear Lady/Sir??

I have been taught, since young, that when writing letters (email), we always start with "Dear Sir/Madam". However, my younger colleague came in and he always start his email with "Dear Lady/Sir".

Which format is more acceptable, and more formal? Please advise. Thanks.

I think Dear Madam/Sir is more appropriate.

I use 'Dear Sir or Madam'. Don't use 'Lady' unless you are writing to someone with a title, like 'Baroness', in which case you say 'Dear Lady X'

Thanks, Doush, Tdol.

Doush, would you like to elaborate more why "Dear Madam/Sir" is more appropriate as compare to "Dear Sir/Madam"?

I would not agree with that order. ;-)