How to change Drupal's default "Unable to send e-mail." 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.