Where does Mail store its email files

Question

hi there, I'm having a bit of an email crisis.
background: I have a MacBook Pro circa 2012 running OS X Yosemite 10.10.2 with over 28 or 29GB of free space.
For email, I have a primary address through Cox Cable and some secondary Cox Cable email accounts for junk mail and the like.  Recently, there was some recurring difficulty retrieving the Cox emails.  The Mail program would open up and look for the Cox Cable email accounts and variously fail to connect with any of them.  The Mail program would tell me that it was unable to access the account and suggest I enter my password.
I decided to delete the accounts and set them up again to see if doing so would grab the most current SMTP settings, etc..  Doing so, I may have accidentally deleted from local storage (on my computer) my old archive of sent and received mail (going back to 2006).  More on that later.
When establishing the settings in Mail for the cox accounts, I left the feature on where Mail would leave the the emails on my Cox email server (which only uses 7% of its space currently).  This seemed to correct the retrieval errors I was getting before.  Something else strange happened.  Sometimes when I would send mail, it would send, I'd hear the whooshing sound, but it would not store a copy of it in my sent email folder.  In most cases, the recipients confirmed receipt.  This would only happen with some sent mail.  Others would be deposited into the Sent mail folder in Mail as usual.
Questions:
(1) Where does OS X store the Mail email files?  I can't find any folder with an xbox file or any other file.
(2) How can I find out if my missing email archive doesn't exist somewhere else on my laptop right now and my Mail program can't find it.
(2) If I well and truly deleted the archive not only from my Mail program but also from my laptop, am I able to import my email archive from my external hard drive where it was backed up?  I'd like to leave the archive in an archive folder in Mail so that I can see it and search it when necessary without having to plug in my external drive and also so there are two copies of this email archive.
Any advice you can provide will be very much appreciated.
Thanks!
ed

Answer 1

hi.  update.  I found the hidden Library folder where the mbox email files are located as follows:
click on your finder to open it
click the GO command from the drop down menu and before clicking again press and hold down the option key.  that reveals in the Go drop-down menu the hidden Library folder.  Click on that Library folder to open it.
from there, look for a sub folder called Mail
click on that and see a sub folder called V2. 
click on that and you'll find subfolders for the email accounts you've established, inside which are the .mbox files that represent those email databases. 
however, once inside there, I found no .mbox files for the old archived files and so I think that I goofed and well and truly deleted the old POP emails when switching from POP to IMAP about two weeks back.
the good news is that I was backing up somewhat regularly via Time Machine to an external drive.  So I recovered the old POP emails into an separate INBOX and SENT MAIL archive that now sits again on my Mac in my Mail program, somewhere below the active mail folders I use daily. 
The POP email protocol was lately prone to these 'unable to access' and 'please enter your password' messages and that's what prompted me to try to change my settings.  When I did, I must have found that Cox email now supports IMAP for residential customers and made the switch from POP to IMAP, inadvertently deleting the old POP email.
The IMAP protocol resolved the 'unable to access' and 'please enter your password' messages that Mail used to give me under the POP protocol.  But left the problem of certain sent mails not being stored immediately in my local sent mail folder on my Mac.
unresolved questions are:
i generally liked switching from POP to IMAP because my email on my phone and laptop are more apt to be in sync and coordinated, and it resolved the connection/password problems I had with POP.  I'd like to keep on IMAP.  I have experienced the occasional missing sent mail.  So far as I can tell, when sending the email from my laptop, I hear the swoosh of sent mail and the recipients do receive it but a copy if not always delivered into my local sent mail folder.  a copy of it is stored on the Cox email server, however. 
any workaround that issue?  It leaves me wondering sometimes whether the mail was sent or received and I also need a record of all email sent on my laptop for future reference. 
is it possible this missing sent mail only happens when there are attachments?  it doesn't happen every time.  I've noticed it has happened when I've sent emails with attachments.
any help you can give is appreciated!
ed