Several new plugins/modules implemented

[sblinch]

I've made some additions to Maddy that were essential for my own deployment; I'm not sure if any of these would be useful to anyone else, or appropriate for the Maddy core, but I thought I'd offer them up for review just in case. I wouldn't consider them heavily battle-tested, but they've been in production for about 6 months on my own low-volume mail server.

Complete change list is here.

Some highlights:

• pattern check plugin, to whitelist/blacklist senders, recipients, hosts, and headers by pattern, regexp, or CIDR
• spamassassin check plugin, adding SA support
• geobl check plugin, to ban countries by connecting IP
• domainbl check plugin, to support SURBL/URIBL/etc
• safelist support to add "safelist" in addition to "reject", "ignore", etc., which overrides all other check plugins and ensures a message is delivered
• crypto storage module, a wrapper that transparently adds encryption support to any other storage blob module
• delivery attempt logging, to more easily grep for specific delivery incidents in production logs
• dnsbl improvements to log which dnsbl providers returned hits, and why and be more resiliant to dnsbl outages
• table storage blob module, to use any table module (such as sqlite) as blob storage
• require_matching_rdns_lax check plugin, handles a commonly-encountered corner case for require_matching_rdns (not sure if this is even relevant any more; I see there were some changes involving this code since I originally implemented it)

For documentation, for now, I've included a doc.go for each plugin/module that's formatted similarly to docs/reference/checks/*.md.

[foxcpp]

crypto storage module, a wrapper that transparently adds encryption support to any other storage blob module

Note that while it protects full message contents, storage.imapsql saves a lot of metadata in its database directly, therefore leaking information about the message, e.g. subject, sender, date, content-types.

[foxcpp]

I will go over these in detail this weekend and see how (if) these can integrated into core.

[sblinch]

storage.imapsql saves a lot of metadata in its database directly

Right! I should probably better document this. My use-case (which since changed, as I ended up needing to switch back to Dovecot for IMAP) was to host Maddy on a trusted server with full-disk encryption, but to use less-trusted S3 buckets for blob storage. This was just intended to add a crypto layer to Maddy's existing S3 support, but I built it as a module wrapper as it could be useful for other future remote blob storage modules.

I will go over these in detail this weekend and see how (if) these can integrated into core.

Sounds great.