How to change Drupal's default "Unable to send e-mail." message? I wanted to override/change the text message that appears while sending Drupal mail.

This is the text "Unable to send e-mail. Please contact the site administrator if the problem persists."

What are the ways to change this message into my own custom message ?
You can use String Overrides modules for overriding any string that passed through t().
Here are some details about this module from its project page:

Provides a quick and easy way to replace any text on the site.
Features:

Easily replace anything that's passed through t()
Locale support, allowing you to override strings in any language
Ability to import/export *.po files, for easy migration from the Locale module
Note that this is not a replacement to Locale as having thousands of overrides can cause more pain then benefit. Use this only if you need a few easy text changes.