I am a little puzzle on this relay thing.

I am using Slack 9 and sendmail 8.12.9

My domain myhoo.net is residing on this server.

On the LAN - I can send/recv email.

On the road - I can only recev email.
Sending email out will get 550 relay denied becuase I am connecting from my ISP dialup domain. Apparently it did not check the FROM address but check the relay address which is my ISP address.

I have add the domain myhoo.net on this files
/etc/mail/local-host-names, relay-domains, access

If I put my ISP domain in the /etc/relay-domains, I will be able to send out the emails. However, this will also allow everyone from the ISP to send out email from my domain right?

Is there another way around this?

Most ISP's block you from using another smtp server, this ensures and protects them from users on their networks to spam others using other smtp servers not on their network.

You could possibly setup your smtp server as your outgoing server for mail but usually you have to specify it authenticate you for it to work properly, basically prompting for a username and password for it to use, like if you were recieving mail.

You can configure authentication for smtp or specify only IPs you allow relay from (your local IP+maybe IPs from your local network).