Working:
astolfo.email (tested by Niko)
cock.li (tested by me, Niko, and prints)
Disroot (tested by Niko)
HEY (tested by sladewatkins)
int.pl (tested by Niko)
IONOS (tested by XP Luna)
Posteo (tested by Niko)
Spaces (tested by notterra)
Proton Mail (tested by aeoe and XP-x64-Lover) (No direct link since honeypot)
Tutanota (tested by Niko) (No direct link since honeypot)
Yahoo (tested by Darth Agnon, dvincent, and Tyorgg) (No direct link since big company willing to sell data)
So far I have been able to fix the issues with these, however I do not recommend them due to having this issue in the first place:
FORTHnet (tested by GoodConcscience)
Horizon Mail (tested by Niko)
Gmail
GMX/Mail.com (tested by me and OCCiera)
Vivaldi Mail (tested by Niko)
Broken:
Hotmail
iCloud
Outlook
Yandex?
Edit: Server IP appears to be on Proofpoint blacklist, so email services that don't use Proofpoint should work. Added a bunch more services that failed to send in mass email.
Edit 2: I have gotten emails sending to Gmail, GMX, and more. The below will be left for historical purposes.
I've noticed recently that emails going to GMX addresses never get received and there's no error or rejection email sent to Eclipse. Gmail on the other hand said today that they've blacklisted the IP of the Eclipse server. I don't know why this is happening. I don't know what they have to gain by blocking Eclipse. Maybe old enemies are still angry and made it happen. I don't know.
I've only been able to test one other email service and that is cock.li. The name may be funny, but it's one of the few free email services I would actually trust. Not run by some big company or corporation hungry for your data. They still receive emails from Eclipse just fine.
So far sample size of three showing the bigger email services blocking Eclipse. I don't know what happened or what I would've done for this to occur. I don't think Eclipse is big enough for them to collude or has done anything. I don't know what the reason is for this.