Multiple email clients sharing one email base

[Timothy Ha]

I have a large email base with over 100,000 emails in several accounts, 
compiled in several years. Recently I successfully converted the base 
from The Bat (a popular email client made in Russia, for Windows) to 
Mozilla Thunderbird. I am very pleased with Thunderbird.

But I also want to try living in Evolution to have a calendar and 
organize my work schedule. What is the best way to share email between 
Mozilla Thunderbird and Evolution without converting to and from one 
client to another?

I know I can put everything to a local IMAP server and browse the base 
with any client. But is there another way to share?

Where and what format does Evolution use to store its emails?

Timothy.