What exactly you meant with "I can't scan it"?
About the time it takes to scan a file or folder, it varies also depending on the type of file. For example, most known video files will be scanned very quickly.
Avast will start checking the file, and if it determines that the file can't be of any type of problem, Avast goes on, no matter the file's size. Why waste time on checking / scanning something that has no means / way / method to activate some malware?
It is possible that the files you were scanning are such that they can't represent any problem.
BTW, when you "download the email", do you mean that you are downloading an actual file attached, a link to a "web email", or an email file (like *.eml, for example)?
Of course some setting may be skipping those specific files, but if you haven't configured it to do so, it is probably nothing to worry about (as long as Avast is active and the shields are working).