How to move large emails from one mailbox to another on Exchange 2010

I'd like to move some emails from one mailbox to another. I have both mailboxes accessible in Outlook 2010. All emails have moved successfully except a couple that are >20mB, for those I receive:

Cannot move the items. The item cannot be moved. It was either already moved or deleted, or access was denied.

Since it's just the larger emails that won't move I keep looking toward limits as being a culprit, except there aren't any set. The recipient mailboxes are set to use database defaults, and the database storage limits remain off. (Since they're off I haven't changed them, but I'm aware it can take up to 2 hours for changes to apply unless the Information Store service is restarted.)

I'm not even sure if I'm looking in the right area. Since all the other emails from the same folder continue to move perfectly fine I can't imagine it's a permission issue. What could it be, where should I look?

I'm not an Exchange expert, if you need more information ask and I will provide. This is Exchange 2010 SP2. Other emails with similar attachments (in number and type) moved correctly but weren't as large, it really does seem to be size-related.