Learn English – “don’t” vs “didn’t”

It depends on what the relationship is like with the recipient. If you are fairly friendly you can be less formal.

Assuming you include the e-mail you are referring to in some way, you might say:

Have you had a chance to look at this yet?

This informally conveys the sense that you know how busy they are.

For a more formal version you would probably want to write it as a full e-mail:

John/Sir/Whatever you'd normally say

I was wondering if you have had a chance to look at the e-mail below yet? [Give a reason] Our customer enquired about it earlier today.

Many thanks,

Bob

Both are correct.

The second is using a form of elision that is only appropriate in some contexts like notices and headlines, but since this text is presumably in a notice, that's okay.

If this was part of a larger piece of running text, that would not be okay.

If this was followed by a larger piece of text, then it likely would be okay. Indeed it would be ideal; such headline-style notes are good for small pieces of text in a larger font or otherwise made particularly prominent, then followed by more detailed text that gives a fuller description. The elision of some words helps it stand out (one of the reasons the style is also used in headlines).