Can I change my iCloud email as I have got married and want to use my new surname
How do I change my iCloud email address all devices
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The Apple ID say firstandlastname@gmail.com is set up in Mac / iPhone / iPad is permanent and its suffix the surname can't be changed .
You have to create a new Apple ID using desired mail services How to create a new Apple ID - Apple Support
From Mac change the primary Apple ID to new one from link appleid.apple.com see the article Change your Apple ID - Apple Support
From another devices like iPhone / other Mac / iPad the changed Apple ID address has to be updated see the article What to do after you change your Apple ID or password - Apple Support