Does anyone else get complaints from some of their portal users that emails are losing links?

This is happening on really core emails like password resets.  Most users will receive a perfectly good link, but some just see plain text instead.  And when they forward the email, the link has vanished.

Yahoo users seem to be particularly affected but it is not just them.  I've had it reported on both desktops and phones.  Any ideas what settings I should tell people to look at please?