Learn English – How should I (or Should I) reply to ‘Enjoy’ mail

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

The traditional English reply to thanks is to say "you're welcome" or various informal variants of that "it was nothing", "think nothing of it", "oh, any time", "my pleasure" and so on.

More generally, I think you need care less about that bit of the conversation. "You're welcome" suffices to reply to the thanks, but when someone has been sick, and enquiry after their well-being and expressing the hope that they are recovered should be the main part of your communiqué. But that stretches beyond questions about EL&U.