mx_check always results in MX_INVALID

[Peter Wyss]

I am currently installing rspamd 1.7.9 on a Debian 9 server. Everything is working except the mx_check module. It always results in MX_INVALID, except when whitelisted. I already tried increasing the timeout, but I don't think it is a timeout issue. Enabling debug for the module does not produce any additional log messages. Enabling the debug logging for rspamd also does not create any additional information regarding that module. I know the module is enabled, as whitelisted domains are properly excluded from the mx check.

Does anyone know what the problem could be or how I could debug it? I ran out of ideas where to look.