With Microsoft Exchange server all information is stored on the server itself, the only thing that is stored on your iOS device are the message headers and a few of the most recent messages that you have recently read.
Once you delete an email account (of any kind) from the iPhone the message headers and messages stored there are deleted. I suppose that if you have a backup of your device from before that time on your Mac you could use one of the many iOS backup extraction apps might be able to get some of that info off of the backup, assuming it is not encrypted.
And like IconDaemon pointed out you have no legal right to the messages without the permission of your former employer. The best way to get needed info out of emails is to contact them and request the specific messages you need.