Stale mails in leap-mx mail spool

on antelope, there are several stale mails:

root@antelope:~# ls /var/mail/leap-mx/Maildir/new/ -la
total 44
drwx------ 2 leap-mx leap-mx 4096 Feb 24 19:24 .
drwx------ 5 leap-mx leap-mx   36 Feb  4 12:57 ..
-rw------- 1 leap-mx leap-mx 3753 Feb 23 00:06 1456185975.Vca01I181a8014M204971.antelope
-rw------- 1 leap-mx leap-mx 3487 Feb 23 00:11 1456186271.Vca01I181a801eM738910.antelope
-rw------- 1 leap-mx leap-mx 4060 Feb 23 00:11 1456186271.Vca01I181a801fM848469.antelope
-rw------- 1 leap-mx leap-mx 3707 Feb 23 00:16 1456186568.Vca01I181a8023M402046.antelope
-rw------- 1 leap-mx leap-mx 4961 Feb 23 03:01 1456196469.Vca01I181d8020M408293.antelope
-rw------- 1 leap-mx leap-mx 4280 Feb 23 03:16 1456197370.Vca01I181d8024M320567.antelope
-rw------- 1 leap-mx leap-mx 4445 Feb 23 14:47 1456238847.Vca01I181258aeM898387.antelope

meskio, could you pls have a look why they weren't delivered ?

(from redmine: created on 2016-02-24, closed on 2016-05-31, relates #7961 (closed), relates #7998 (closed))