saving sent mail in sent folder

[soumitri at iitk.ac.in]

Hello,

I am getting following error in info log of dovecot.

imap(someshb): Info: Connection closed (No commands sent) in=0 out=374

This error is for some users (not all) while sending email  via 
Thunderbird. The email is sent but not saved int the Sent folder.

I tried to debug the issue and getting nothing except following

Debug: Effective uid=2121, gid=4619, home=/users02/home/ppd_doaa

imap(ppd_doaa): Debug: Namespace inbox: type=private, prefix=INBOX., 
sep=., inbox=yes, hidden=no, list=yes, subscriptions=yes 
location=maildir:~/Maildir:INDEX=/xx/index:CONTROL=/xx/index

imap(ppd_doaa): Debug: maildir++: root=/xx/Maildir, index=/xx/index, 
indexpvt=, control=/xx/index, inbox=/xx/Maildir, alt=

imap(ppd_doaa): Debug: Namespace : type=private, prefix=, sep=, 
inbox=no, hidden=yes, list=no, subscriptions=no location=fail::LAYOUT=none

imap(ppd_doaa): Debug: none: root=, index=, indexpvt=, control=, inbox=, 
alt=

namespace has following

namespace inbox {
   hidden = no
   inbox = yes
   location =
   mailbox Drafts {
     special_use = \Drafts
   }
   mailbox Junk {
     special_use = \Junk
   }
   mailbox Sent {
     special_use = \Sent
   }
   mailbox "Sent Messages" {
     special_use = \Sent
   }
   mailbox Trash {
     special_use = \Trash
   }
   prefix = INBOX.
   separator = .
   type = private
}

i am using dovecot-2.2.36-10.el8.x86_64

Thanks for any help.

-- 
Sincerely,
Soumitri