commit a8ccdaf109b85cedfd609678d95f7b7d5fdb91f5
Author: ellie timoney <ellie@fastmail.com>
Date:   Mon Jun 7 12:00:27 2021 +1000

    lmtpd: shared mailboxes don't have conversations to lock
    
    Fixes #3488

diff --git a/imap/lmtpd.c b/imap/lmtpd.c
index 498bb8765..dc9ca21bc 100644
--- a/imap/lmtpd.c
+++ b/imap/lmtpd.c
@@ -843,8 +843,10 @@ int deliver(message_data_t *msgdata, char *authuser,
             // lock conversations for the duration of delivery, so nothing else can read
             // the state of any mailbox while the delivery is half done
             struct conversations_state *state = NULL;
-            r = conversations_open_user(mbname_userid(mbname), 0/*shared*/, &state);
-            if (r) goto setstatus;
+            if (mbname_userid(mbname)) {
+                r = conversations_open_user(mbname_userid(mbname), 0/*shared*/, &state);
+                if (r) goto setstatus;
+            }
 
             /* local mailbox */
             mydata.cur_rcpt = n;