You use 'regarding' to introduce something you are going to talk about. You use 'considering' to take something into account, before talking about something broader that encompasses it.

1) "Regarding the weather conditions today, it is going to rain."

It means that I'm going to talk about the weather now, and that I know it is going to rain today.

2) "Considering the weather conditions today, it is going to rain."

It means that when I take the weather conditions into account (cloudy skies, cold temperature, windy weather), I can take a guess that it is going to rain today.

Hi Aryan,

When we consider something, we are thinking, contemplating and reflecting about it for the purpose to reaching a decision. 

Example: I considered myself fortunate to have a job during the recession. (reflection). I have considered my options very carefully and decided to maintain the status quo. (Decision)

When we use regarding, we also consider something. 

Example: I regarded Susana as a close confidante. 

We also use "regarding" to mean with reference to something. 

Example:

Regarding your request, I'm pleased to inform you that it has been approved. 

As you can see, they are used in different contexts.

Hope this helps.

Cheers, Lance 

Yes, I would definitely say #1 is the winner, although it would be "Regarding *the* e-mail...". 

If you give an example context / sentance, I can help tell you which would be correct.

To give a very simple idea:

Regarding = about

Considering = thinking about

There really isn't any difference, they are both formal words that mean just about the same thing.

It would be more correct to say ESPECIALLY (not specially) in business conversations and emails.