Most likely the email you’re seeing rejected has yahoo.com in the From header. While we do our best to support email forwarding, Yahoo has made a demand in their DMARC record:
➜ ~ dig TXT _dmarc.yahoo.com +short
“v=DMARC1; p=reject; pct=100; rua=mailto:dmarc_y_rua@yahoo.com;”
What this means is that if an email is received with yahoo.com in the From header, and the sender doesn’t match their SPF record, then they request that the email be rejected. All major email services have begun respecting this. Yahoo does not add other email services to their SPF record, meaning the only approved sender for yahoo.com is Yahoo. Anyone else sending email as yahoo.com will be rejected. When it passes through our servers, we become the sender.
While SRS is a standard that was supposed to solve this issue, and we do utilize it, SRS does not actually change the “From” header and SRS is not respected by any of the major email services when DMARC records specify “reject” in them.
Unnecessary and excessive rejections hurt IP reputation, which makes your emails more likely to end up in spam folders than inboxes. To prevent unnecessary rejections, we intentionally do not send email that we know will not be accepted, therefore we do not send email that has “yahoo.com” in the From header.
In short, Yahoo doesn’t want their email forwarded, and it won’t be accepted by any major email provider if we try to, so we don’t forward it.