What Mail Files to back up in Server Folder

I am trying to figure out, particularly, what files do not need to backed up. This is because the files that appear to be index files are very large and frequently changing. Can anyone tell me which of the following files, located in each user's mailbox, need not/should not be backed up?

dovecot-uidlist

dovecot-uidvalidity

dovecot.index

dovecot.index.cache

dovecot.index.log

dovecot.index.log.2

dovecot.skindex

maildirsize

dovecot.skindex is particularly problematic, because it is huge and always changing. I have tried to research it and, unfortunately, have failed to find anything informative about this file. Any information would be much appreciated.