Let's encrypt DNS problem: NXDOMAIN looking up MX for server

When try to create a new SSL-certificate, I get the following error (see below)
The strange thing is that NXDOMAIN looking up MX for server, and server is not the full server name, that should be some server.domain.com
So that's why server can't be found I think but where can I change server into the server name (server.domain.com)??

I'm using:
DA 1.53.4
Let's encrypt 1.1.10
Nginx 1.14.0 (reverse proxy)

Cannot Execute Your Request

Details

Generating 4096 bit RSA key for let's encrypt account...
openssl genrsa 4096 > "/usr/local/directadmin/conf/letsencrypt.key"
Generating RSA private key, 4096 bit long modulus
.............................................................++
.....++
e is 65537 (0x10001)
Account registration error. Response: HTTP/1.1 100 Continue
Expires: Mon, 27 Aug 2018 10:54:14 GMT
Cache-Control: max-age=0, no-cache, no-store
Pragma: no-cache

HTTP/1.1 400 Bad Request
Server: nginx
Content-Type: application/problem+json
Content-Length: 164
Replay-Nonce: jG6tOA2eqFOQ4N4Yq4RIh-nBJoowrCUBYA44kqXw_tM
Expires: Mon, 27 Aug 2018 10:54:14 GMT
Cache-Control: max-age=0, no-cache, no-store
Pragma: no-cache
Date: Mon, 27 Aug 2018 10:54:14 GMT
Connection: close

{
"type": "urn:ietfarams:acme:error:invalidEmail",
"detail": "Error creating new account :: DNS problem: NXDOMAIN looking up MX for server",
"status": 400
}.

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