In reply to Paul Phillips - UKC and UKH:
The change to yahoo.com was made a few years ago, which stopped emails that failed DMARC (combination of spf and dkim) from being delivered. Earlier this year yahoo rolled our the change to their .co.uk addresses as well. At some point this year google will probably implement the same policy as well.
This has a big effect on anything which sends email "on behalf" of someone else, such as the old ukc contact form, or club mailing lists using mailman.