When trying to send mail from your server, messages never arrive, and you possibly receive bounce-back messages stating the message was undeliverable.
You can typically receive mail and can send mail to other addresses on your server, though.
Why is this happening?
Many providers block access to send data out on port 25 as a way to prevent spam being sent from their network, but, as this port is essential for communicating with external mail services, it also prevents mail from sending for legitimate users.
You can review further instructions on identifying this issue and workarounds available via this article.