Add information about antispam technologies

According to the NÚKIB rules for e-mail technologies, we have to apply some security measures starting at Jan 1st, 2023. These measures include antispam technologies (SPF, DKIM, DMARC) configured to be strict/enforcing. It may cause problems in the cases where notification messages are sent to addresses forwarded/redirected to other addresses because such forwardings violate the defined rules. Such messages need not to be delivered if target servers enforce these rules.

Thus I've added a special warning about this problem. The warning is both in the reForis section and the FAQ section. Mailing lists are also mentioned because they may or need not work; it depends on their implementation and configuration (e.g. which headers are overritten or whether message bodies are modified).