dmarc.Lookup() merges multiple TXT records into a single string

[wttw]

net.LookupTXT() returns a slice of strings, with one string for each TXT resource record returned. If there are multiple strings inside a TXT record it concatenates those before returning it.

dmarc.LookupWithOptions then concatenates all of those TXT resource records into a single string before parsing it.

If there are multiple TXT records for _dmarc.example.com it'll likely give the wrong response. That's not going to happen in
a healthy DNS zone, but it happens occasionally.

I think that correct behaviour would be for dmarc.LookupWithOptions to remove any TXT records that don't start with "v=", then return an error if there are more than one, then parse just the first remaining TXT RR, if any.

[emersion]

Section 6.6.3 says that we should indeed ignore TXT records that do not start with v=.

[emersion]

This actually causes more issues: #62