Use IMAP
Try using IMAP email on both your iPhone and mac.
One of the most frustrating things is I'll read an email, then go back to it (even when it's still on my server) and it is no longer readable. This seems ot be because the phone downloads each message each time you try to read it, even if it downladed it before. This wastes time (particularly on Edge), and more important, it means I have to wait to download mail into my computer until I've fully exhausted any views I may want on the phone.
With my Treo, I'd download to the phone, then download to my computer, and the messages could be read in either place. Not so with iPhone.
Apple needs to wake up and realize that most people will not be using WIFI to retrieve emails. With Edge, they need to download it only once.
Try using IMAP email on both your iPhone and mac.