Which one is correct to say?
I have sent her an email with instruction about how to add mailboxes into her outlook
I have sent her an email about how to add mailboxes into her outlook.
The second sentence implies that the e-mail contains no other significant content. Those instructions are the entire point of the e-mail.
The first sentence doesn't have this implication. Those instructions are included, but other things might be included as well.
If the e-mail does include other things, the first sentence is a better choice.
The phrasing "instructions about" doesn't sound quite natural to me. It isn't wrong, but I would recommend "instructions for adding" as a better option than "instructions about how to add".