Backing up 100's of mailboxes?

[Dal123]

I have approximately 100 'mailboxes' in my apple mail, this number is only going to increase.
I only know how to export mailboxes as a way of backing up my emails. This is not very practical with the amount of emails. What are my options?
I plan to get an external hd soon and use time machine for back-ups. What are my options until then as a fail-safe?

 

[maflynn]

Manually copying the mailboxes from your ~/Library

Your post begs the question as to why in the world do you need 100s of mailboxes. Without knowing the details, it seems a bit over-kill to create that many

 

[Dal123]

I need that many mailboxes because I run my own construction business. It's a specialist business and there are many, many emails I need to keep dating back at least six years for legal reasons. Better to keep all information as some areas you need to keep documents for at least ten years I think e.g. building specifications, drawings etc.