dovecot - Sep 2014 - numeric username

Heya,

does dovecot support "numeric only" usernames (in format
"^0-9+$")?

We are unable to deliver emails to such usernames via LMTP:
Sep 22 08:41:05 razor4 postfix/lmtp[2120]: D4FD91CDC11F: 
to=<231588 at example.org>, relay=example.org[private/dovecot-lmtp], 
delay=217653, delays=217653/0.01/0.02/0, dsn=4.2.0, status=deferred 
(host example.org[private/dovecot-lmtp] said: 451 4.2.0 
<231588 at example.org> Permission denied (in reply to end of DATA
command))

On normal username format (^[a-z][-a-z0-9]+$) everything is working fine.

Permissions are the same as on other accounts:
# ls -la .mailbox
-rw-------+ 1 231588 users 0 Sep 19 20:13 .mailbox

Using dovecot 2.2.13 with local accounts on RHEL and (relevant) configs:
auth_username_chars = 
abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ01234567890.-_@
auth_username_format = %Ln
auth_username_translation ...
lmtp_address_translate ...
passdb {
   args = cache_key=%u%s dovecot
   default_fields    deny = no
   driver = pam
   master = no
   name    override_fields    pass = no
   result_failure = continue
   result_internalfail = continue
   result_success = return-ok
   skip = never
}
...
service lmtp {
   chroot    client_limit = 1
   drop_priv_before_exec = no
   executable = lmtp
   extra_groups    group    idle_kill = 0
   privileged_group    process_limit = 0
   process_min_avail = 5
   protocol = lmtp
   service_count = 0
   type    unix_listener /var/spool/postfix/private/dovecot-lmtp {
     group = postfix
     mode = 0600
     user = postfix
   }
   unix_listener lmtp {
     group      mode = 0666
     user    }
   user    vsz_limit = 18446744073709551615 B
}
....
userdb {
   args    default_fields    driver = passwd
   name    override_fields    result_failure = continue
   result_internalfail = continue
   result_success = return-ok
   skip = never
}

Thank you in advance,

cheers, Jernej