Welcome to the neighborhood
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Your server "should" just send mail out of the box. You maybe need to add your domain into the virtual email domains though. When you originally set your server up you set a domain (local like Zentyal.lan) and a default email domain. If you left it all stock then you have only the local domain, so all of the users would have email addresses like "
john.doe@zentyal.lan" or something.
So let's say you bought the domain "example.com", if you login to the admin panel and click on Mail > Virtual Domains then you can add your "example.com" domain into it. After that you have 2 options for configuration.
1. Give all of the users an "alias" for sending/receiving public emails
2. Delete each users existing email address - BEWARE this will delete everything in their mailboxes.
To do either go into Users and Computers > Manage and click on each user. You'll see their email address right there for changing if that's the route you want to go. Underneath the "Change" button you'll see "Modules Configuration" and a link for something like "Mail Account Settings" (not sure what exactly that one says) but you can click to expand it. You'll then see the section to add an alias, type it in and click the + button.
There is a possibility that your ISP is blocking port 25 as well. They just do that a lot these days. If you've already put your top level domain into the virtual domains and it isn't delivering mail outside of your office then I'd guess that is the culprit right there.
Finally, if you're using a router/firewall device to connect the server to the internet you'll have to do some port forwarding to your server. Make sure the router is directing the following ports to your servers IP address:
1. MAPI - 135
2. SMTP - 25
3. HTTPS - 443
4. HTTP - 80
If none of that works for you come post back and I'll try and get back to you on it.