NXDOMAIN error on part of hostname

When creating a new SSL-certificate, I get the following error, see below:
The problem I think is the hostname, because it's not the full hostname. It's the hostname without the domain name. (Look at the red text, it has to be something like server.domain.com)
My question is "Where can I change the hostname from server into server.domain.com, because then the NXDOMAIN can be found".

Thanx for your reply!

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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 07:42:13 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: sOTyrTDN2HrDKXOd8xN3M9hMfrSm65qTkvtogw6zoQs
Expires: Mon, 27 Aug 2018 07:42:13 GMT
Cache-Control: max-age=0, no-cache, no-store
Pragma: no-cache
Date: Mon, 27 Aug 2018 07:42:13 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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