Unable to receive or send email on iPhone 6s

For a few days now, I haven't been able to receive messages on my iPhone 6s and when trying to open what messages I did have prior to a few days ago, I get the following message, "This message has not been downloaded from the server." However, intermittently, I am able to open one of those emails. I also get a message like, "...the user name or password for imap.aol.com is incorrect..." What steps do I need to take to resolve this problem? Any support is greatly appreciated.