I see this isn't the first time this issue has come up! But it looks like the last time was 9 years ago, so...

I run my own email server so I see all the gruesome details about spam filtering. I use the Rspamd filter, which is one of the two big (non-Gmail) spam filters out there, and it absolutely hates DhO's subscribed-thread notification emails. Here's a typical filtering breakdown:

[7.45 / 15.00] 
 HFILTER_HELO_5(3.00)[DhOUbuntuServer] 
 FORGED_RECIPIENTS(2.00)[m:noreply@dharmaoverground.org,s:eric@ericsemailaddress.org] 
 RBL_SPAMHAUS_CSS(2.00)[159.69.90.185:from] 
 RBL_VIRUSFREE_BOTNET(2.00)[159.69.90.185:from] 

So that's the total message spam likelihood (7.45, higher number = more likely spam) followed by the individual rules that triggered, and how much they contributed to the score (I've omitted several little ones that don't contribute much and can't be helped).

HFILTER_HELO_5: This means that the DhO mail server is identifying itself as "DhOUbuntuServer", but that's not a real domain name, much less the actual domain name that handles DhO's mail. The actual domain name looks like it's "smtp.secureserver.net". It looks like DhO is using the postfix mail server, so the solution there is to set "myhostname = smtp.secureserver.net" in postfix's "main.cf" configuration file.

FORGED_RECIPIENTS: I'm not sure what you do about this. The problem is that the "To" header is set to "noreply@dharmaoverground.org", but it's actually sent to "eric@ericsemailaddress.org" (not my real address ). Presumably the "To" should be set to the address it's being sent to.

RBL_SPAMHAUS_CSS: This is a fairly big blacklist site, and it's worth getting off their list. Instructions are here: https://www.spamhaus.org/sbl/delistingprocedure/

RBL_VIRUSFREE_BOTNET: I hadn't heard of this one. Their website is here: https://www.virusfree.cz/en/, but I wasn't able to find instructions about getting delisted.

I hope this information might be of use to someone! Obviously this is my own email server, but these are default settings for one of the biggest spam filter packages out there, so I think it's worth taking seriously. At the very least, the HELO issue and Spamhaus listing are going to get DhO dinged with every mail server out there.

Let me know if I can provide any more information.

Eric, I'm getting an admin to address this. Stay tuned, please.
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Thanks for the quick response!

Of these issues the HELO thing is the most important, but also the most confusing. I can promise that "DhOUbuntuServer" is not the right value, and likely comes from the host server name being used as a default value when "myhostname" is not set, but I'm not confident of the right value.

"smtp.secureserver.net" and "mailstore1.secureserver.net" are both advertised as handling mail for "dharmaoverground.org", and the originating IP address seems to be 159.69.90.185. One way or another, those values need to be correctly pointed at one another via DNS, and then "myhostname" needs to be set accordingly. I've always found the whole thing baffling, however, and can remember how it works for less than ten minutes before it runs out of my brain.

Are you receiving email from DhO now, Eric?

Yessir! Score is now 4.4, under the 5-point threshold (for my system, anyway).

It looks like we're off the "virusfree" blocklist, but the other issues still remain: still on the Spamhaus list, the forged recipients still an issue, and while the HELO is fixed, it's now showing me "HFILTER_HOSTNAME_5(3.00)[static.185.90.69.159.clients.your-server.de]", so there' still something funky going on with the hostname and IP address.

But it's better! If we could get off Spamhaus at least, that would give the emails a fighting chance.

Thanks for looking into this!

On a possibly related note: 

This is the 3rd or maybe 4th account I made here, and the first one for which I get any emails (I abandoned the previous ones due to no way to do password recovery).

I don't use automatic spam filtering and I checked the spam folders. I figured that maybe DhO doesn't send emails to German email addresses (my current account is made with a Gmail address but the previous ones all were with popular German email address providers).

Here are some instructions for managing your own notification and email settings on DhO (credit to Siavash):

To start or stop email notifications for the entire message board, on the recent posts page, click on your avatar icon in the upper right-hand corner beside the search box, and select Notifications.

On the Notifications page, hover the mouse below the Welcome title in the upper left part and above the Notifications title. It should show a blue border around that box with an overflow button (three vertical dots), click that and select Configurations.

On the next page, you can select on or off for each of the notifications and whether to have them as email or on the site and hope that it'll work.

There are a lot of choices, so read through them carefully, especially the ones under the heading "Message Boards."