Mbox locking info update

.github/actions/spelling/expect.txt

@@ -16,6 +16,7 @@ Amavis
 amavisd
 AND'ing
 apparmor
+appendfile
 APPENDUID
 APY
 arcfour

docs/core/config/mailbox/formats/mbox.md

@@ -155,6 +155,12 @@ mbox_write_locks = dotlock fcntl
 
 Here's a list of how to find out the locking settings for other software:
 
+#### Debian
+
+Debian's policy specifies that all software should use "fcntl and then
+dotlock" locking. All software in Debian packages are supposed to be patched to
+use this order by default.
+
 #### Procmail
 
 ```console
@@ -175,24 +181,28 @@ virtual_mailbox_lock = fcntl
 ```
 
 In the above case, if you used the mailbox transport, you'd have to change
-Dovecot's configuration to [[setting,mbox_write_locks,fcntl]] dotlock or
+Dovecot's configuration to [[setting,mbox_write_locks,fcntl dotlock]] or
 vice versa for Postfix.
 
 If you used the virtual transport, it doesn't really matter if the dotlock
 is missing, since the fcntl is common with Dovecot and Postfix.
 
+#### Exim
+
+Locks are controlled by the appendfile transport's `use_lockfile`,
+`use_fcntl_lock` and `use_flock_lock` settings. The default is to use
+lockfile (dotlock) and then fcntl lock.
+
+#### Sendmail
+
+Sendmail's mail.local delivery always uses dotlock followed by flock.
+
 #### mutt
 
 ```console
 $ mutt -v|grep -i lock
 ```
 
-#### Debian
-
-Debian's policy specifies that all software should use "fcntl and then
-dotlock" locking, but this probably applies only to most commonly used
-software.
-
 ## Directory Structure
 
 By default, when listing mailboxes, Dovecot simply assumes that all files it