The file that was infected was apparently essential to my ability to send and receive mail via the MAIL application on MacBook Pro. Can I reinstall the file? If so, will I be able to disinfect it? Thank you!
avast! for Mac OS X does not delete files by itself, it must be explicitly done by the user. In other words, if You have deleted a file and did not made a backup of it, there is almost no chance to get the file back.