Three ways, and they are not the same

• Messages: talking to one member. It is a conversation: they reply from their area and the whole thread stays.
• Announcements: an email to a group. For meeting calls and things that must be read.
• Notices: the noticeboard. It shows in the members area and sends no email at all.

Which to use

If it does not deserve an email to two hundred people, it is a notice. Emailing about everything is the fastest way to make people stop reading them.

If a member writes

Answer, even if only to say you are looking into it. Somebody who writes and gets no reply does not write again, and then you find things out in the bar.

Before the first email

If none go out, it is almost always the sender address. Tell whoever administers the site: there is a warning in the panel that explains it and says where to change it.