messages disappeared from some mailboxes and smart mailboxes following removal of an account from Mail
Removing an account from Mail removes all local copies of messages for that account. 


  
  Is there any way to retrieve all my emails? (Do I still have the old email's folder somewhere?)
  


If the account was for an e-mail service that uses IMAP or Exchange Web Services

In Mail, simply recreate the account. With IMAP, all messages: 


are (primarily) stored on the server
may be (secondarily) cached by e-mail clients such as Mail in OS X


– and so recreation of the account should regain access to everything that is on the server. 

If the account was for an e-mail service that uses POP

It's normal for Post Office Protocol (POP) to not leave a copy of a message on the server after that message has been delivered. (Metaphorically: your postman or postwoman does not keep copies of your mail.) However, e-mail client software may be configured to stray from that default, to leave some things on a POP server. 

If you used POP for your account configuration: 


be prepared for the possibility of not finding some things on the POP server
it may be desirable to use Mail on your other Mac (the Mac Pro) to save or export data.


If the deleted messages were on the iMac alone, and if you had no backup on a separate disk

The likelihood of recovering data from JHFS+ may be low. 

OS X tends to quickly reuse space after that space is marked as free, especially if that space is an an area of hard disk that is preferred for performance. 

Mobile Time Machine local snapshots provide handy backups, but this feature of Time Machine is laptop-oriented (probably not enabled on your iMac).