Mail Proxy Servers
Yahoo Mail Proxy is a content fetch proxy that retrieves the page content of URLs that are embedded within emails sent to Yahoo Mail users. Having the content displayed through the proxy improves the security for our users while reducing overall network usage.
Web site access for Yahoo Mail Proxy is identified by “YahooMailProxy” User-agents. The full user-agent value you'll see in your access logs are:
Requests to images and potentially other assets embedded in emails sent to our users will come from the following IPs. We recommend not to rate-limit or block those to ensure recipients of your emails do not have a broken experience.
27.XXX - 98.XXX
106.XXX - 216.XXX
2001:XXX - 2a00:XXX
In addition you can find the latest list of IPs published here