Index: openacs-4/packages/acs-mail-lite/tcl/acs-mail-lite-procs.tcl
===================================================================
RCS file: /usr/local/cvsroot/openacs-4/packages/acs-mail-lite/tcl/acs-mail-lite-procs.tcl,v
diff -u -r1.108.2.26 -r1.108.2.27
--- openacs-4/packages/acs-mail-lite/tcl/acs-mail-lite-procs.tcl	16 Mar 2022 14:36:24 -0000	1.108.2.26
+++ openacs-4/packages/acs-mail-lite/tcl/acs-mail-lite-procs.tcl	16 Mar 2022 15:28:48 -0000	1.108.2.27
@@ -389,12 +389,12 @@
 
         ad_try {
             db_foreach get_queued_messages {} {
-                # check if record is already there and free to use
-                set return_id [db_string get_queued_message {} -default -1]
-                if {$return_id == $id} {
-                    # lock this record for exclusive use
-                    set locking_server [ad_url]
-                    db_dml lock_queued_message {}
+                # Lock the message and also make sure that it was not
+                # locked or deleted by somebody else in the meantime.
+                set locking_server [ad_url]
+                db_dml lock_queued_message {}
+
+                if {[db_resultrows] == 1} {
                     # send the mail
                     ad_try {
                         acs_mail_lite::send_immediately \
@@ -415,10 +415,15 @@
                             -extraheaders $extraheaders \
                             -use_sender_p $use_sender_p
                     } on error {errorMsg} {
-                        ad_log Error "Could not send queued mail (message $return_id): $errorMsg"
-                        # release the lock (MS not now)
-                        # set locking_server ""
-                        # db_dml lock_queued_message {}
+                        ad_log Error "Could not send queued mail (message $id): $errorMsg"
+                        # Uncommenting the following two lines would
+                        # cause failed emails to be retried at the
+                        # next sweep. This could make sense only for
+                        # certain kinds of errors, so it is best left
+                        # commented and delegated to e.g. downstream
+                        # callbacks.
+                        ## set locking_server ""
+                        ## db_dml lock_queued_message {}
                     } on ok {r} {
                         # mail was sent, delete the queue entry
                         db_dml delete_queue_entry {}
Index: openacs-4/packages/acs-mail-lite/tcl/acs-mail-lite-procs.xql
===================================================================
RCS file: /usr/local/cvsroot/openacs-4/packages/acs-mail-lite/tcl/acs-mail-lite-procs.xql,v
diff -u -r1.19.2.2 -r1.19.2.3
--- openacs-4/packages/acs-mail-lite/tcl/acs-mail-lite-procs.xql	2 Mar 2021 12:01:30 -0000	1.19.2.2
+++ openacs-4/packages/acs-mail-lite/tcl/acs-mail-lite-procs.xql	16 Mar 2022 15:28:48 -0000	1.19.2.3
@@ -1,19 +1,12 @@
 <?xml version="1.0"?>
 <queryset>
 
-    <fullquery name="acs_mail_lite::sweeper.get_queued_message">
-        <querytext>
-            select message_id as id
-            from acs_mail_lite_queue
-            where message_id=:id and (locking_server = '' or locking_server is NULL)
-        </querytext>
-    </fullquery>
-
     <fullquery name="acs_mail_lite::sweeper.lock_queued_message">
         <querytext>
             update acs_mail_lite_queue
                set locking_server = :locking_server
-            where message_id=:id
+             where message_id = :id
+               and locking_server is NULL
         </querytext>
     </fullquery>