imap4s timeouts and closed connections

[tech-lists]

Hi,

context: Mutt 1.12.1 / freebsd12-stable

I'm using fastmail for all my mail and I access it with imap4s. Lately
I've been seeing timeouts, or a mailbox will just close [1], and yet my
connection (fibre) is stable. I've been using imap with mutt ever since 
direct imap support became available, and this is the first time I've
really had any problem with mutt in an imap context.

I have the following in my .muttrc:

set mail_check=90
set imap_keepalive=30
set timeout=15

I can be just reading mail in the mailbox then "connection closed"

How can I fix? I've done some searching about this but can't find much
for my version of mutt and what I did find was inconclusive. But it's an
issue that seems to come up repeatedly.

[1] doubly annoying because my lastread isn't recorded ie the mailbox
closes without synching.

thanks,
-- 
J.
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