Incorrect message size reported using IMAP with a Pine client
Synopsis: In order for Exchange to return the exact size, it must rebuild the MIME which can have a performance impact on large messages or deeply nested attachments. The default setting is to return an estimate of the size to improve performance and since most other IMAP clients don't have any problems if the size is wrong.
You can enable the exact size for everyone with:
Or only for a specific user:
Set-CASMailbox "IMAP User" -ImapUseProtocolDefaults:$false -ImapEnableExactRFC822Size:$true