Attempting to install an Apple application via a '.pkg' can cause more problems then such attempts to resolve. Explaining anything more is beyond the intended scope of this reply.
'Mail' on the 'Dock' is merely an alias.
01. Navigate to the 'Applications' folder ('/Applications/').
02. Double click on 'Mail'.
If 'Mail' launches, then the 'Dock' based alias needs to be removed from the 'Dock', and 'Mail' (in the '/Applications/' folder) dragged onto the 'Dock'.
If 'Mail' does not launch - there are various possible resolutions.
Try the following:
01. Create a new user account via the 'System Preferences' 'Accounts' utility.
02. Log into the new user account.
03. Click on 'Mail' - on the 'Dock', or double click on 'Mail' in the '/Applications/' folder.
If 'Mail' launches, then you confirmed that 'Mail' (the applications) works, and it may be a corrupted preferences file ('com.apple.mail.plist', in the '~/Library/Preferences/' folder) and / or a damaged item or items in the 'Mail' and / or 'Mail Downloads' folder(s) (in the '~/Library/' folder) of the original user account.
If 'Mail' does not launch - the original 'Mail' cannot launch problem remains, and / or the attempted installation of 'Mail' created a problem.
Report your observations to the above processes, for possible further assistance.
