Restricting sending mail to domain or group

[admin]

Hi folks,

has anybody a simple solution for the following request?

I have a group alias (all at company.com).
(1) Only company.com accounts should be able to send an email to everybody in that company via all at company.com.
(2) - rather optional: refine the restrictions, e.g. two groups, grp1 at company.com and grp2 at company.com. Grp1 members should be able to send mails to grp2 but not vice versa.

What else than sieve would keep dealing with that demands as simple as possible?

Thanks in advance and sorry for abusing this list for this kind of „problem“ which is not a bug.

Regards,
-M