Problems with corrupted mail on Macbook

Hi - I have a recently purchased Macbook Pro running High Sierra. Lately, very strange things have been happening with Apple Mail. I'll receive an email which shows up perfectly normally on my iPhone, but, on the laptop, shows up either as a garbled mish-mash of several e-mails, or - more weirdly - as a perfect rendition of a completely different e-mail that's been previously deleted. I've tried rebuilding my Inbox several times without success, but that's about as far as my expertise goes. Any suggestions greatly appreciated!