You raise an interesting question, but I need a clarification.
The title of your original post is "Client with 50GM Exchange Mailbox." Are you going to implement ActiveSync and Exchange web services and give her the SmarterMail Exchange experience?
If so, and you don't already have those two add-ons licensed, you will have to license them to work with your SmarterMail installation.
Getting back to the question of IMAP vs EXCHANGE, you might want to read this TechNet article from the Exchange Team Blog:
As you will see, it's not the size of the mailbox, but the number of items which seems to the root cause of synchronization issues. According to everything I have read, and seen, IMAP might actually be the better way to handle this account.
This will also be dependent on a number of other factors:
- How will she access her e-mail?
- Does she also want to synch her contacts, calendar, and notes?
- What kind of mobile device operating system will she use?
- Windows Phone
- Firefox OS
- Sailfish OS
- Ubuntu Touch OS
- What other clients will she be accessing her e-mail with?
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