I've seen several quick, web-based configuration testing tools for clients and servers of various protocols, so it could potentially be helpful to someone if I compiled a list of them here. These tools test not only that your servers are working, but that they are configured in accordance with best practices, and I use them all heavily. Let's start with TLS, DNS and SMTP, though I'll try to find more in future and append them here.
DNS — authority
- intoDNS by Elvsoft
- DNS check tool by Pingdom
- DNSViz by Sandia National Laboratories
- DNSSEC Analyzer by Verisign Labs
DNS — resolver
SMTP
- Authentication Checker by Port25
- DomainKeys, DKIM, SPF, SpamAssassin Email Validator by Brandon Checketts
- High Assurance Domain DMARC testing tool by NIST
- DMARC Record Assistant by Kitterman Technical Services
- DMARC Inspector by Jason Bela and Tim Draegen