Ok, thanks Alexander, so I'm still in process of creating the mail server. I've had steps forward, but a couple back, as I updated systems at in opportune time and lost apache2. Working on that. I looked at the nullmailer install. I like the date on it: 2018. (Not too old, not too new.) Read the HOWTO to glean what I could.

Those were very helpful links. I'll repost this one: Installing_and_Configuring_Postfix_on_Debian.

Your comments made me realize that the problem space here was wall-to-wall debian. I'll go ahead and write this like we used to on usenet when the subject became implementation-specific:

[specific to debian implementations]

I had been unable to completely update my droplet until I found a bit of extra update syntax from ubuntu. I crammed it all into a new alias:

I had been in a limbo zone where I almost had 22.04 but now do:

root@fourth:~# release
...
Checking for a new Ubuntu release
There is no development version of an LTS available.
To upgrade to the latest non-LTS development release 
set Prompt=normal in /etc/update-manager/release-upgrades.
root@fourth:~# apt autoremove
[download]

Ok, and thanks for your comment. I'm happy to see your responses as I know they are likely to contain useful and exotic details of what is happening underneath the hood with unix. One issue I need to get clear is whether Digital Ocean is giving me port 25, so I will fire off this to them:

Dear Digital Ocean Support, I've been renting space from you for over a year now, and, because google will disallow 3rd-party use of their smtp server on May 30, am working up an email server with my DO site. I'm not entirely convinced that I have access to port 25, as it is blocked during the first 2 months. Do I? Thank you for your attention,

Beyond this, I got some partial results and hope to not need to start over. I think I got the MX record set up right with DO. I had to access the way back machine for some of my sources. This is from 5 years ago: way back machine on mx records.

Here's a nice, new fresh one: how-to-install-and-configure-postfix-on-ubuntu-22-04. I'll end up going through this again. Since the beginning of this write-up, I tore out and replaced apache2, changing Postfix twice, so I'll be due to go over it again.

One final thing. I told a friend that google was restricting 3rd-party access to their smtp server, and she asked me whether her iOS mail app that gets her gmail will still be able to do so. Does anyone have the expertise to speculate? (I hemmed and hawed.)

One issue I need to get clear is whether Digital Ocean is giving me port 25

Very simple to test this for yourself, but do note that most virtual host providers block SMTP traffic. Log into your server, and run the netcat command, listening on port 25 (need sudo because the port number is less-than 1024).

steve@cesium:~$ sudo nc -l 25
[download]

Now, on a remote machine, using netcat again, create a connection to the server, and type something, and hit ENTER:

steve@maezi ~ >nc cesium 25
hello, world!
[download]

If port 25 is open on your server, you'll see your typed message on its console:

steve@cesium:~$ sudo nc -l 25
hello, world!
[download]

You can also type something while on the server while the nc command is still running on both machines, and the message will show up on the machine you connected to the server with; in essence, a little chat program ;)

hey stevie, looks like I'm getting through from the local machine:

$ nc HOSTNAME 25
hello, email world
hey stevie, it worked!
^C
$
[download]

to the remote target:

Last login: Thu May 19 00:12:40 2022 
wilma@fourth:~$ sudo -i
[sudo] password for wilma: 
root@fourth:~# which nc
/usr/bin/nc
root@fourth:~# sudo nc -l 25
hello, email world
hey stevie, it worked!
root@fourth:~#
[download]

You can also type something while on the server while the nc command is still running on both machines, and the message will show up on the machine you connected to the server with; in essence, a little chat program ;)

Yeah, I guess so.:) Reminds me of the old days when we really used telnet....