Is Linode blocking port 25?
No. Computers on most residential networks are blocked from sending mail both on the sending end (e.g. your ISP blocking port 25)"
MX Toolbox is showing my SMTP as blocked - 25 smtp Refused (RST) 0
yet, when I enter ufw status it shows 25 ALLOW Anywhere
__More Information About Smtp Connect
We attempted a connection to your mail server on port 25 but your server did not respond. More than likely there is no email server at this address or you have a misconfigured firewall or spam filtering system.
If the SMTP Connect test is unable to connect to your mail server after 15 seconds we will report it as an error. For optimal email deliverability it is advisable that SMTP connections should not take longer than 15 seconds.__
Outgoing Mail (SMTP 587/postfix) is running but is not publicly accessible at LindodeIPAddress:587.
They will usually allow you to connect to their mail servers on port 25, but not to anyone elses mailserver (including your linode) on port 25.
If you have a linode, you can run your mail server there. Linode do not block port 25.
Your linode mail server will send mail to other mail servers on port 25, and will receive mail from other mail servers on port 25.
Now to send mail to your linode when connected to the internet through your ISP, you will need to send mail to the mail server on port 587, (because your IPS is blocking port 25) which is the submission port.
The submission of mail to mail your mail server should be setup to require authenication.
For this to work your mail server will need to be configured to accept mail on port 587, and you will need to configure some kind of authentication (username / password) or you will be an open relay, and the world will hate you. Although the spammers will love you, and you don't want that.
Quick update: we've implemented a new policy that blocks outbound connections on ports 25, 465, and 587 blocked by default. More details in our blog post: A New Policy to Help Fight Spam