Frances, thank you. Hadn't thought to do that. But they were all set at Unlimited attachment size. The two who replied both have older IPhones. I asked them to check and see if they were trying to attach JPEG or HEIC format files. One is pretty capable, the other gave me a chuckle when she replied that she doesn't speak Greek.?
Anyway. it looks like things have now "cleared" up. So I'm back to my original guess that it was an issue with their phones.
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Pat P
Pat Pratt