I have been having issues sending photo attachments using Samsung Email and Google Photos. I had remote support who just said I need to resize attached images to bypass an issue. However, I have since realised that if I attached through Samsung Gallery then there are no issues and the photos send fine and open with no issues by the recipient
How are your photos stored in Google Photos? If they are uncompressed that may be the reason they won't send. I think Samsung Email has a file size limit. The Gallery app may resize them automatically.
I personally could never get to grips with Samsung EMail, it's far too clunky and fussy for me. Gmail is much better in my opinion.
I have a Sky email address, which is primarily why I use Samsung email. It also works on my Samsung Watch. I don't remember having this issue before. I use Google Photos are they are automatically backed up.
