write a how to debug mailing issues

[noave]

• check for .qmail files
• check spamfolder status
• check sieve status
• check MX records
• check spf, dkim
• show what we need for support questions: specific mailaddresses, timestamps within last two days for logs

[quite]

Yeah a way to get logs about what sieve decided to do with a mail would be good... like legacy ~/var/log/mailfilter.log