On the whole the language used in e-mails should be conversational in tone, i.e. informal and personal, but still professional.

Abbreviations (eg. I'll, he isn't) are normally used in e-mails.

Another method for keeping the tone conversational is to drop the subject occasionally:

eg. Hello Michael,
Hope you had a good holiday...

We often use the word "just":
eg. Hello Michael,
I just wanted to let you know ...

It's a good idea to make the mail personal in some way, with a sentence referring to your colleague's holiday / weekend / work. This should either be at the end or at the beginning of your e-mail. Here are some examples off typical sentences you might use:

Hope your holiday / presentation / meeting went well.

It's good to have you back at work.

How are things in London?

Looking forward to seeing you next month.

There are different possibilities for closing your e-mail:

Best regards is a more formal ending, whereas Best wishes is quite personal. If your mail closes with a friendly sentence, eg. Have a good week, then it's also possible just to close with your name.

We tend to use a lot of will + infinitive in mails when we offer to do things:

I'll send you a copy as soon as it's ready.

I'll call the supplier next week.

I'll make sure the equipment is in the room when you arrive.

We also use the present perfect when explaining steps we have taken:

I've finished the report and faxed it to the supplier.

The present continuous is often used to describe fixed plans:

We're holding the meeting next Tuesday.

I'm taking the 7.30 flight from Frankfurt.