Thanks for reporting this.
I dug into how the "Check DNS" feature works, and
good news: it's actually unrelated to your SMTP settings.
What "Check DNS" does
It's a pure diagnostic that queries your site's domain for SPF/DKIM/DMARC/MX
records via PHP's dns_get_record(). It doesn't send any email — that's a
separate "Test Email" button.
Why you're getting "Invalid response from server"
That message means the check endpoint didn't return valid JSON — usually
because dns_get_record() is disabled or outbound DNS lookups are blocked by
your host (common on shared hosting), or the request timed out. A couple of
things to check:
- Ask your host if
dns_get_record() is enabled and outbound DNS (port 53)
isn't firewalled. - Check your server's PHP error log for a fatal error at the moment you click
"Check DNS".
About missing verification emails
This might actually be a side effect of the same DNS issue, not a separate
email/SMTP bug. Flatboard has a setting, "Vérification DNS (MX) des
emails" (Settings → Email), enabled by default — it checks the new
registrant's email domain has an MX record, and blocks registration
outright if the DNS lookup fails. If DNS resolution is broken on your host,
this could be silently rejecting signups before an account is even created.
Suggested steps
- Go to Settings → Email and temporarily disable "Vérification DNS (MX) des
emails" — see if registrations start working. - Click "Test Email" (not "Check DNS") to confirm your SMTP/mail delivery
actually works independently. - Check your Flatboard logs for
PHPMailer or mail() errors around the
time someone registered — send failures are logged server-side but not
currently shown to the user.
Let us know what you find, especially your hosting provider — this really
sounds like a hosting-level DNS/network restriction rather than a Flatboard
config issue.
