So we came across a situation where we had a client with over 75gbs of email data in Our Mail server configured as activesync.

Webmail stopped loading, due to the size of the email account im assuming.

We were trying to use outlook to migrate the data out into a .pst
BUT outlook turns grey and keeps crashing and freezing. After 2 days of this we finally decided to start looking for Tools or data convertors.
So we found this . We downloaded the trial and attempted a export and it WORKED! and fast as well.
https://www.recoverytools.com/mailenable/migrator/buy.html
We purchased the $500 copy. Lifetime license. We were then able to open the exported .pst's into outlook without any issue.
Again this was to convert a 70gb email account from the mail servers raw data into an outlook.pst file.
Hopefully this helps someone out there.
Thanks
Graham - DMS
www.dmsservices.ca
- below is the version we purchased.
Migration
Suitable for Enterprise Use
Valid for All Machines
Convert Unlimited Mailboxes
Batch Conversion Support
Lifetime License
Free 24x7 Support
Saving Options :
PST, MSG, EML, EMLX, TXT, MBOX, HTML, MHT, XPS, RTF, DOC, Windows Live Mail, ICS, VCARD, CSV, PDF, Lotus Notes, Zimbra, Maildir, Thunderbird, OLM, Office 365, Outlook.com, Exchange Server, Gmail, GSuite, Yahoo, IMAP