Sending messages to domains without MX records fails

[foxcpp]

Describe the bug

Something goes wrong when maddy tries to use non-MX hostname.

Log files

remote: cannot use MX   {"domain":"aaa","msg_id":"f2423444-5fd6ff38","reason":"dns: domain must be fully qualified","remote_server":"aaa","tls_err":null}
queue: delivery attempt failed  {"domain":"aaa","msg_id":"f2423444","rcpt":"mysh@aaa","reason":"dns: domain must be fully qualified","smtp_code":451,"smtp_enchcode":"5.4.0","smtp_msg":"No usable MXs, last err: dns: domain must be fully qualified","target":"remote","tls_err":null}
queue: not delivered, permanent error   {"msg_id":"f2423444","rcpt":"mysh@aaa"}

Environment information

• maddy version: 0.4.3

[foxcpp]

miekg/dns library returns an error if domain passed to dns.TLSAName is not a FQDN. Per discussion in #311 we just force it to be one. This is affecting only cases where there are no MX records since domains in MX records are FQDNs.

Thanks for the report!