luceos
You are correct (of course). I reiterate the following only in the interest of helping relative newbies like myself having trouble with finding a clear solution via Google. (It's hard to know what to search when you don't know what software layer is causing the problem.)
As best as I can tell, setting SMTP_BLOCK in CSF firewall (the default firewall in DirectAdmin) to "0" (instead of "1") clears out the "out of the box" default block placed by DirectAdmin and CSF on all three email ports -- 25, 465 and 587.
The button 'Disable SMTP_BLOCK' can be found by clicking the button 'Fix Common Problems' which is under the "ConfigServer Security & Firewall" tab, which itself is under the "Extra Features" tab, which is in the "Admin" view in the DirectAdmin GUI. The button's text area does not explain what ports it re-opens, but it seems to cover all three.
When speaking to multiple server hosts, they all spoke of possible blocks on their servers on port 25. (Of course they could also impose blocks on 465 and 587 but they never referred to those ports in my discussions).
You just can't assume that cPanel and DirectAdmin have the same default behaviors. My research indicates that at least as of a few years ago, cPanel does not automatically block email ports (or have its default firewall do so).
The state of online documentation is such that at least 95% of the support postings provide complex, manual command line methods for editing blocked ports in CSF firewall (once you realize that is the problem), without even realizing that there is a one-click solution in DirectAdmin for opening the CSF blocks on email ports.
This is probably due to the beautiful solution for this very common issue being buried many layers deep in the DirectAdmin GUI.
