New features

• Experimental: multithreaded validation. The most difficult part of ordering ACME certificates is to provide proof of ownership for the host name(s) that are to be included. The validation process can take a decent amount of time because services like Let's Encrypt have to be thorough to maintain the trust of the international community. For example, they examine the challenge answers from multiple locations around the world to ensure that some localized network level attack doesn't allow hackers to illegally obtain a certificate. There is nothing we can do to speed up a single validation, but for certificates with multiple host names, we can. We currently validate each host name in serial order, which means there is a lot of unnecessary waiting, because each validation is in theory completely independant of any others. Using the new DisableMultiThreading setting you can now opt in to parallel validation, meaning that your SAN certificates will validate much faster (the setting is named like that because multithreading should become the new default in a future release).
• A similar new feature is batch preparation and cleanup which allows plugins to work more efficiently during the stages before and after validation. DNS validation plugins that want to support multithreaded validation need to be able to manage multiple active TXT records. In some cases it's possible to create and delete these records using a single call instead of one by one, providing an additional performance win. So far this has only been implemented for Azure plugin.

Enhancements

• #1586 - Additional parameters have been made available for custom DNS validation script. Specifically there is {ZoneName} which is replaced with the registerable domain, and {NodeName} which is the part of the {RecordName} remaining after stripping off the registerable domain (or @ they are equal). Requested by @hlsantos.
• #1602 - Searching by friendly name is no longer case sensitive, requested by @Smurgl

Bug fixes

• #1567 - After sending an email, the connection with the SMTP server was not nicely closed, reported by @hlsanton
• #1568 - We were a too strict on the ACME standard for DigiCert, reported by @Stan-tastic.
• #1569 - Interactive mode now respects command line arguments as overrides for the global defaults, reported by @DamienLaw
• #1578 - The Route53 plugin would crash when multiple zones have been configured for the same host name. It would also potentially attempt to update private zones, which are ignored now, reported by @tsimmons
• #1591 - The default path setting for the PfxFile store plugin was not applied, reported by @cutig3r
• #1593 - The scheduled task health check use case sensitive method of checking the path, reported by @CriteriaFirst
• #1600 - Fix crash when IIS is detected in the registry but not actually installed, reported by @darkworks
• #1603 - Fix mixup between CentralSsl path and PemFiles path introduces in 2.1.8, thanks @yndtrud
• #1605 - Fix annoying "Invalid anti-replay nonce" bug - reported by @morhans and others.