To help prevent spammers from abusing your domain, you can add SPF record to your zone.
Use SPF Wizard to construct the record. Result should be put into TXT record.
For BIND, simply add line into your zone file:
mydomain.com. IN TXT "resulting spf line"
Also for each of your MX records, add line:
mx.mydomain.com. IN TXT "v=spf1 a -all"
where mx.mydomain.com is your MX name
For tinydns, add resulting lines to your root file (usually /etc/tinydns/root/data) and then run make.
If you run two servers don't forget to refresh the second one.
To check your domain zone you can use DNS Report, it will perform thorough check and alert you of all warnings and errors.