Emails Ending up in Junk

[Asai]

Greetings,

I'm trying to debug an issue with a user who is finding emails (user 
uses Thunderbird as client) ending up in Junk folder, and constantly 
marks email as Not Junk, but important email ends up in Junk folder.

I have sieve set up, but am unable to determine if this is the result of 
a sieve problem (sieve will automatically move certain emails into Junk 
if certain headers are present, but these headers are not present on 
these emails.)

I'm looking in Dovecot debug logs to try to figure this out, but all I 
can find is sometimes this, and I don't know if this is relevant.  Can 
anyone point us in the right direction for solving this issue?

Apr 01 11:51:05 imap(user at domain.tld)<102535><+kTsYd6+f83AqEbH>: Debug: Mailbox Junk: Mailbox opened because: UID move
Apr 01 11:57:08 imap(user at domain.tld)<102735><P3dSa96+kM3AqEbH>: Debug: Mailbox Junk: UID 565: Opened mail because: prefetch
Apr 01 11:57:08 imap(user at domain.tld)<102735><P3dSa96+kM3AqEbH>: Debug: Mailbox Junk: UID 565: Opened mail because: access
Apr 01 11:57:08 imap(user at domain.tld)<102735><P3dSa96+kM3AqEbH>: Debug: Mailbox Junk: UID 565: Opened mail because: full mail
Apr 01 12:00:20 imap(user at domain.tld)<71333><OX7P1+2+BfcKCgFy>: Debug: Mailbox Junk: Mailbox opened because: STATUS
Apr 01 18:52:10 imap(user at domain.tld)<114066><WLt3mPO+z8QKCgFy>: Debug: Mailbox Junk: Mailbox opened because: SELECT

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