On line 44 of the module you have:
drupal_add_css(drupal_get_path('module', 'uc_regions') .'/uc_regions.css');There is no such file in the distribution, and all this works without any error, until you try and use the mimemail.module, which assume all the files added by 'drupal_add_css()' exists - and an error is generated.
While mimemail.module needs to do better checks against the file system, I suggest to remove that line if the CSS file does not exist.