In reply to Postmanpat:
> (In reply to Philip)
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> [...]
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> Possibly, bizarrely they don't officially support iPads or iPhones, probably the two most used devices to receive e mail in the world.
In your world perhaps, and I'm sure apple would like you to believe that. In reality they are a tiny proportion of devices accesing email.
In answer to your conundrum, I would guess that one of your devices is downloading and deleting messages from the server. On each device check your IMAP/POP3 settings, and you should find a check box that says "leave a copy of messages on the server" make sure this is checked on all devices.
I've used Yahoo for years on multiple devices and never had a problem once I'd made sure the setting was correct.
The only other thing I would suspect is that your mailbox is over the maximum allowed size and its deleting big emails, but I would presume that yahoo would have checked that first
Post edited at 16:37