Please see:
https://help.yahoo.com/kb/SLN24016.html
Why are my emails bouncing?
Yahoo recently updated the DMARC record with "p=reject" for the "yahoo.com"domain.
This means all DMARC compliant mail receivers (including Yahoo, Hotmail, and Gmail) are now bouncing emails sent as "@yahoo.com" addresses that aren't sent through Yahoo servers."What does Yahoo recommend I do?
We recommend ESPs ensure all messages can be authenticated by DKIM
and/or SPF. To achieve this, ESPs should use domains that they're
authorized to send emails from. It's not recommended to use a customer's
personal domain.
To receive responses with an "@yahoo.com" address, either a Reply-To:
header or email forwarding from authoritative domains are the suggested
options. Bounces should also be managed within your authoritative
domains.
Yahoo's SMTP service is designed to send user emails only, and it can't be used as a replacement.
Visit our Yahoo Mail Tumblr page for more information, including further steps you can take to fix this problem."
Does the domain upon which your Moodle server runs have SPF records or DKIM or DMARC records which would include your Moodle server?
@john ... you could ask your students to use another free EMail address - like Google's gmail.
That has an un-intended possible consequence .... discovering what one could use with Google Docs in Moodle.
'spirit of sharing', Ken