qmailでdigコマンドなどではメールサーバは正常に稼動しているように見えるけど、but_connection_died と帰ってくる

qmailのバージョン

qmail-1.03

配送ログ

Apr  5 02:49:30 web qmail: 1238867370.102192 starting delivery 100222: msg 3189356 to remote xxxx@xxxx.com
Apr  5 02:49:47 web qmail: 1238867387.323540 delivery 100222: deferral: Connected_to_xxx.xxx.xxx.xxx_but_connection_died._(#4.4.2)/

MX情報

$ dig @10.20.138.9 xxxx.com MX               

; <<>> DiG 9.3.4-P1.1 <<>> @10.20.138.9 xxxx.com MX
; (1 server found)
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 43165
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 2, ADDITIONAL: 2

;; QUESTION SECTION:
;xxxx.com.                        IN      MX

;; ANSWER SECTION:
xxxx.com.         86400   IN      MX      10 yyyyy.com.

;; AUTHORITY SECTION:
xxxx.com.         86400   IN      NS      yyyyy.com.
xxxx.com.         86400   IN      NS      yyyyy.com.

;; ADDITIONAL SECTION:
yyyyy.com.     1800    IN      A       iii.iii.iii.iii
yyyyy.com.     1800    IN      A       iii.iii.iii.iii

;; Query time: 30 msec
;; SERVER: 10.20.138.9#53(10.20.138.9)
;; WHEN: Fri Apr 10 18:29:15 2009
;; MSG SIZE  rcvd: 131

メールサーバの情報

$ dig @10.20.138.9 yyyyy.com ANY

; <<>> DiG 9.3.4-P1.1 <<>> @10.20.138.9 yyyyy.com ANY
; (1 server found)
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 47502
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 2, ADDITIONAL: 0

;; QUESTION SECTION:
;yyyyy.com.            IN      ANY

;; ANSWER SECTION:
yyyyy.com.     1633    IN      A       sss.sss.sss.sss

;; AUTHORITY SECTION:
yyyyy.com.            1633    IN      NS      zzz.com.
yyyyy.com.            1633    IN      NS      qqq.com.

;; Query time: 9 msec
;; SERVER: 10.20.138.9#53(10.20.138.9)
;; WHEN: Fri Apr 10 18:32:02 2009
;; MSG SIZE  rcvd: 99