I updated my main BBS to A47C from 2/12 and it looks to me files are causing the busy issues I'm seeing:
----------------- MUTIL v1.12 A47 2021/02/12 Sun, Feb 21 2021 (loglevel 3)
+ Feb 21 19:04:29 Startup using mailin.ini
- Feb 21 19:04:29 EXEC ImportEchoMail
- Feb 21 19:04:29 EXEC LinkMessages
- Feb 21 19:04:29 EXEC FileToss
+ Feb 21 19:04:29 Process: Toss FDN/TIC Files
+ Feb 21 19:04:29 Waiting for BUSY nodes

So it looks to me like junk in /mystic/files/badfiles and I should likely just delete everything in the badfiles, does that sound right to you?
Most of it seems to be from years past and I deleted it once before, but that was before I upgraded to the 2/12 alpha, but it's re-appeared. Not sure why,

--- Mystic BBS v1.12 A47 2021/02/12 (Raspberry Pi/32)
* Origin: Mystic Pi BBS bcw142.zapto.org (21:1/145)

On 21 Feb 2021 at 07:23p, bcw142 pondered and said...

+ Feb 21 19:04:29 Process: Toss FDN/TIC Files
+ Feb 21 19:04:29 Waiting for BUSY nodes

So it looks to me like junk in /mystic/files/badfiles and I should
likely just delete everything in the badfiles, does that sound right to you? Most of it seems to be from years past and I deleted it once
before, but that was before I upgraded to the 2/12 alpha, but it's re-appeared. Not sure why,

stuff in your bad files dir should not be causing a bsy issue.. but either
way if you wish to delete those files it's entirely up to you. They will have ended up there because of a mix of crc issues, missing tic files, bad
passwords etc.

Have you tried shutting down your server, and running fidopoll killbusy all ?

Then try an manual run of mutil using your mailin and mailout ini files to
see what happens.

--- Mystic BBS v1.12 A46 2020/08/26 (Windows/32)
* Origin: Agency BBS | Dunedin, New Zealand | agency.bbs.nz (21:1/101)

On 25 Feb 2021, Avon said the following...

stuff in your bad files dir should not be causing a bsy issue.. but
either way if you wish to delete those files it's entirely up to you.
They will have ended up there because of a mix of crc issues, missing
tic files, bad passwords etc.

Have you tried shutting down your server, and running fidopoll killbusy all ?

Yes, I did and do so much I've made it a script called busyfix:
#!/bin/bash
# busyfix
# From AVON on fidopoll busy (now ./mis poll busy)
#Chances are you have a ton of unprocessed packets in your BBS inbound.
#
#Check the mutil logs to see if it's reporting things are BUSY so not #processing them.
#If this is the case, stop MIS
sudo killall mis
sleep 10
#and then run the 'fidopoll killbusy all' command
./mis poll killbusy all
sleep 2
./mutil mailin.ini
#see if they process in...
#Then restart MIS
sleep 5
./mutil mailout.ini
sleep 5
sudo ./mis daemon

Just ran it so things are clean right now, but in a few hours it will clog up again. Thinking of adding an Event to check /mystic/echomail/out/*.bsy and
run if it is! The real issue is why? I get that it happens now and again, but with the current setup it's near hourly.

--- Mystic BBS v1.12 A47 2021/02/12 (Raspberry Pi/32)
* Origin: Mystic Pi BBS bcw142.zapto.org:23 (21:1/145)