Before deleting an account in Tiger Mail, the following warning message is provided:
Are you sure you want to remove the (account type here) account “XXXXXXXXXX”?
This will permanently delete the account setup information, mailboxes, and messages from your computer. Copies of messages on the mail server, if they exist, are not affected.
This is usually ignored or not completely understood but is based on the Mail.app filing structure so now the long winded answer for the reason.
Many other email clients utilize a single large database file which is not dependent on individual account and associated mailbox filing. When creating an account in Mail, an account named folder (named by the user name and incoming mail server for the account) is created in the Mail folder at Home > Library > Mail using the Finder. For POP type accounts, the following mailbox (mbox) folders are created within the account named folder when the function is used by the account: INBOX.mbox, Drafts.mbox, Sent Messages.mbox, Deleted Messages.mbox and Junk.mbox (when Junk mail is set to automatic). The mbox folders within the account named folder represent the account named mailboxes under In, Drafts, Sent, Trash and Junk (when Junk mail is set to automatic) in the Mail.app mailboxes drawer.
When deleting an account in Mail, the account named folder at Home > Library > Mail is deleted at that time and as the warning message indicates, the associated account mailboxes within the account named folder are also deleted. You must select OK after the warning message appears.
It would be preferable for the account named folder to be moved to the hard drive Trash can which requires an additional step to delete but this is not what occurs.
To have any hope of recovering your data, you need to immediately stop using your system and then purchase a copy of a recovery tool such as Tech Tool Pro (Micromat) or Data Rescue (Prosoft Engineering), both of which have data recovery tools. I've not used Data Rescue, but I've seen reports that it recovers files that the others cannot, so it might be your best hope. The chances generally aren't really good even so, but short of sending your drive to a data recovery service (expensive), such utilities are your only hope.
Note that the more you use your system before you attempt recovery, the more likely it is that the system will overwrite the sections of the drive where the data you want to recover is stored, rendering it unrecoverable by any means. You should therefore stop using the system completely until you get and try to use one of the file recovery tools.
This also emphasizes the importance of maintaining a regular backup of the Mail folder and other important data.
