If you can get your mailbox into an MS Outlook PST format on your local machine, X1 Search, http://www.x1.com/, works well locally. I am using it to manage my current slice of mail, going back to 2008, about 4.2 GB of mail, about 10K of messages, and the existing PST structure is kept. I have an offline mail store that has been moderately curated with my X1, that takes my possible search scope up to 50K messages, 23 GB of PST's back to Outlook 2000, which was a looong time ago.
Other data sources it can read and index include
- Outlook Email, Contacts, Calendar, Tasks and Notes
- X1 Rapid
Of course, if you have a data source on a network drive or in the cloud, there is a latency factor for building the local index, but the search is fast (especially if you opt to index message content) and the retrieval is fast.
X1 understands over 40+ file formats and can extract all sorts of metadata to help in the search. It can drill down into archive files and index their contents. They also have enterprise solutions and engines.
With 7 TB of local storage on my main desktop, somewhere north of 2,000,000 files, my X1 is essential to my digital life. I just keep adding disk space and copying wholesale drives to bigger drives. X1 helps me keep on top of duplicated and obsolete content. Currently my earliest files date back to 1996 (of course ignoring magic dates like 1-1-1980, 1-1-1970 and 12-31-1969 all that various opsys and file managers filled in ;-)
I've wandered a ways from a way to handle big mail, but I think you should look at X1. And finally, a WooHoo! I just found (while looking at my indexed data for size & count examples), the license information I was needing for a legacy app I run, and with that, I can get my cheap upgrade to the Win10 64bit version. Thanks for asking your question, I found something I needed because of it.