Do the Quest agents have limitation on the maximum amount of messages that can be migrated?
To the current date Support has not encountered any cases where amount of messages in the mailbox causedany issues. Quest agents rely on the Microsoft MAPI APIs for the mail extraction from the mailbox. As such if Microsoft architecture allowed for the mailbox to contain large amount of itemsthese APIs should work as well.
Please also see the following Quest KB where PST size limit of 2 GB was causing a problem during mail migration:
Mail/Public Folder Source Agent stops processing - MAPI_E_DISK_ERROR