Maximum email file capacity?

Jeht

I have a 27 inch iMac and since the last update to 10.10.5 I have noticed that when I forward an email the original is lost.
I save a lot of emails for future use.
how big can the total email file be ? and how do I check the total content ?
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theo

In my preferences I ask mail to create a bcc copy for me, for every new email.
Do you save the emails on your mac or on the server of your account provider ?