The attachments are nearly always jpeg or pdf files.
His emails are usually to just me but sometimes he copies his son and daughter. They do received the attachments just fine.

More relevant information of which he reminded me:
If he sends the emails directly to an @yahoo.com account, I do get the attachments. It is when he sends it to an alumni forwarding account that we have this problem. Emails through @alum.pomona.edu without attachments come through, just those with attachments bounce. I wouldn’t know whether anyone else tries unsuccessfully to send attachments to .edu. IT at Pomona College worked with me on this problem and determined that the emails were going through as expected on their end, that the problem had to be with Yahoo. Of course I can’t communicate effectively with Yahoo (you’ve probably heard this before), so I turned to you. Is this a wrinkle that defies explanation?

I appreciate your trying to help!

Lois (aka Dusty)