It appears that the University of Washington blocking Riseup emails as spam. And, no, I'm not sending spam.
Here are the recent error messages I'm getting:
: host mxe25.s.uw.edu[173.250.227.25] said: 550 5.7.1 198.252.153.8 explicitly prohibited from sending mail for this domain by sender's published policy (in reply to end of DATA command)
: host mxe24.s.uw.edu[173.250.227.24] said: 550 5.7.1 Message not accepted as it appears to be spam (in reply to end of DATA command)
It appears Riseup has been blacklisted by by sorbs.net. See .
19 June 2017 - Riseup IP 198.252.153.8 is still blacklisted by SORBS.
FYI, this ticket was inadvertently cross-posted to [en] General Discussion.

Hello,
We believe we've resolved this problem with SORBS (for now), hopefully things are working properly for you now?
crossbill
We believe we've resolved this problem with SORBS (for now), hopefully things are working properly for you now?
crossbill
Thanks, according to IPINDETAIL.COM, the Riseup IP is delisted now.