Importing external inboxes into Icedove; concatenation isn't working for me

Way back in the old days, I had a *scape inbox that lost its EOF scent. I opened the file with a text editor and [somehow] stopped the scrolling when it got down to nothing but asterisks. I cut them all off, saved the file, and voila ... rescued that inbox. That was in *indows 3.11, I think.

At an intermediate time, when an anti-virus program ate the inbox from the *indows installation of *hunderbird, I was able to snatch the same inbox from a debian installation of Icedove and plop it into the *indows installation of *hunderbird without the slightest hassle or loss of data (I had been keeping the two inboxes synchronized by always downloading messages from first the one computer and then from the other, but only removing them from their servers with the second computer).

Fast forward to the present day. I'm importing inboxes from hither and yon ... and I've gathered a few into one account in my Trisquel installation of Icedove. Icedove steadfastly refuses to open those foreign inboxes (except in one other account where it does so quite happily).

I've concatenated the two inboxes thusly:

> sudo mv Inbox InboxSmall
> sudo cat InboxSmall Inbox01 > InboxAll
> sudo mv InboxAll Inbox

Icedove ignores all but the former contents (i.e. InboxSmall) when opening Inbox after restarting; it does go through some motions.

The byte count of the concatenation satisfies ordinary base-ten mathematics; i.e., Inbox equals the sum of InboxSmall plus Inbox01.

When I try to have Icedove open one of the peripheral inboxes, e.g. Inbox-01, a message briefly pops up, (loosely quoting) "Opening Inbox01 failed; the mail server for that account responded: Mailbox does not exist, or must be subscribed to." Inbox-01 is a closed inbox; I'm not trying to get any more input into it except eventually by concatenating with other inboxes.

Gedit can open either the former Inbox or Inbox-01, except that it complains about some incorrect characters.

In this context, I'm using the term, "scent" to cover either/or in the debate of character vs. integer in EOF's.

Question is: Does Icedove put a secret scent at the end of its own Inbox files ? How can I detect that scent and remove it (presumably) from the intersection of InboxSmall and Inbox-01 inside InboxAll ?