Issue with email on my iPhone

Check the configuration parameters for Apple Mail given by your email provider, it's on the website or in the menu of your email account, then go to general>mail>Accounts, tap on the account that doesn't work, tap on Account, tap on SMTP, tap on SMTP Server and check it matches the configuration given, then tap on done on the upper right corner. Go back to that account tap on Advanced and check also the incoming settings (remember to tap done if you change anything). If everything was fine, it could also be your provider is temporarily down or has problems