If you do not have port 25 open on your mail server's IP address, then this guide is for you.
For example, you're running a custom mail server at home, and your Internet Service Provider ("ISP") has blocked outbound port 25.
Since you cannot have outbound traffic on port 25, then you most likely also won't have inbound traffic on port 25 due to this block.
Assuming you are using our service to forward emails, you can workaround this issue through our FAQ answer here.
If your ISP blocks outbound port 25, then you will have to find an alternate solution or contact them.
You can run
telnet smtp.forwardemail.net 25 from command line or terminal to see if your outbound port 25 connection is blocked.