Hi All,

I dont know if this is the right question for this forum.
I am facing a problem with sending emails to a particular domain,i get error message
as follows,


451 4.4.0 DNS temporary failure (chkuser)

Normally here is what happens, When I send the email first time, it gives above
error, but after resending, it goes through and sometimes it can go after serveral retries.
The domain is accpro.co.bw
My DNS resolution is working fine

dig mx accpro.co.bw

; <<>> DiG 9.9.4-RedHat-9.9.4-61.el7_5.1 <<>> mx accpro.co.bw
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 24547
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;accpro.co.bw. IN MX

;; ANSWER SECTION:
accpro.co.bw. 900 IN MX 0 accpro-co-bw.mail.protection.outlook.com.

;; Query time: 2186 msec
;; SERVER: 154.70.144.68#53(154.70.144.68)
;; WHEN: Thu Sep 12 15:27:54 CAT 2019
;; MSG SIZE rcvd: 86


dig accpro-co-bw.mail.protection.outlook.com.

; <<>> DiG 9.9.4-RedHat-9.9.4-61.el7_5.1 <<>> accpro-co-bw.mail.protection.outlook.com.
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 3092
;; flags: qr rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;accpro-co-bw.mail.protection.outlook.com. IN A

;; ANSWER SECTION:
accpro-co-bw.mail.protection.outlook.com. 10 IN A 104.47.10.36
accpro-co-bw.mail.protection.outlook.com. 10 IN A 104.47.9.36

;; Query time: 3038 msec
;; SERVER: 154.70.144.68#53(154.70.144.68)
;; WHEN: Thu Sep 12 15:29:36 CAT 2019
;; MSG SIZE rcvd: 90



Thank you in advance.

You resolver is too slow and maybe the qmail query times out the 1st time, while it works the next time because the response is cached.
Use a different resolver (like 1.1.1.1, or 8.8.8.8) and see if it's faster.

Regards