Thanks very much for your response to my question, Martin. I guess the protocol behavior of sending smaller messages first somewhat addresses the issue. At least in an Internet-down situation, communications would be possible until one's Inbox fills up and the station stops accepting messages. I don't know when that limit would be hit but probably not for a long time--particularly if the Internet is down. I think the bigger concern would be finding a clear and operational HF station, given all the traffic competing for access while the band is "open."
I have set a pretty strict limit for download size so I won't tie up a station while I sit around wondering how large any prolonged download in progress might be before I must give up and force a disconnect. My understanding about Pat/Winlink forms is that, as is common practice in digital radio, only the contents of the form are sent (via xml) but the form is accessed from a user's local form cache to produce a completed form for viewing on the receiving end.
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Cal -- AD8Q