I've just tested it on what can be considered a fresh install and it seems to work fine sending the mail to my own e-mail server then to me. I didn't do any particular setting up of the box I was testing on.

Hi folks,

Fixed it by installing ssmtp - this works from the command line. No idea why the stock install doesn't work. Thanks to all for your input.

Cheers
Ziggx

It looks like mailx is required by app-smtp-core so it should be installed, as should postfix on a default install. The only point I can see of installing the SMTP server is that you get a webconfig screen to start and stop postfix.

I was wrong about perl. The Mail Settings/Mail Notification app uses a very old version of the Swift PHP mail app, but I don't know beyonf that. I think it has its own mini-smtp server which is why it won't do STARTTLS (until it gets updated).

As Nick indicated you must have a working SMTP server, usually postfix with ClearOS. Also, to use the example command Nick provided you must have the mailx rpm installed. I cannot remember if that rpm is installed by default or whether you must explicitly install it. Use something very similar here to send automated emails from crontab entries at set times.
You can also

cat /some/file | mail -s 'Gateway down' nick@example.com

What does it do when it fails?

You may need to install the SMTP Server app. The Mail Settings app uses some old Perl program to send mail.

Hi Nick,

Unfortunately, that doesn't work - I can't figure what I can send a test message from the web interface but not from a bash script...

Mysterious...

Ziggy

echo 'Gateway down' | mail -s 'Gateway down' nick@example.com

I think you have to have a body (the 'echo' bit of my line) or the mail program hangs waiting for input. It is just possible that you need the SMTP server installed, but I don't think so.