You have defined a NS record (server.7seasent.com.), but you didn't assign it an A record.Try:
@ IN SOA mailsrv.7seasent.com. root.7seasent.com. (
43 ;;serial (d. adams)
1D ) ;;minimum
IN NS server.7seasent.com.
server7 IN A x.x.x.x
mailsrv IN A 192.168.0.204
Replace x.x.x.x with the actual IP of server7 and increase the serial