Without getting to technical, the reason could be your email server settings may have been changed from smtp;simple mail transfer protocol to iMAP. This is controlled by the email provider; Apple, gmail, etc….
One of the differences between the two mail protocols is, that smtp downloads the entire email when you click get mail.
Where the iMAP protocol will only download the mail header info. From, To, Subject title.
You must click the specific email before iMAP downloads the entire message.
This was done sometime ago to help control bandwidth when email was being sent across mail relays via telephone dialups via 9600 baud and up to 19.2k baud.