/ip firewall src-nat add src-address=192.168.1.0/24 action=nat to-dst-address=xxx.yyy.1.1
(xxx.yyy.1.1 being the public address).
does the public ip-address never changes?
on usual dsl-based connections e.g. in germany the ip-address changes at least every 24h, therefore this method won't work without changing the rule manually every day or so.
plus many ISPs do reverse lookups, and for mail exchangers ip-address and hostname have to match.