emails from my iphone are not being sent - they're disappearing

I have had ATT&T for years, same email address.  I have used it on my phone for years.  Recently, I have noticed that I type up a mail, and send it and it completely disappears.  The email is not in Drafts, not in Sent box, and never sent.  My recipients are NOT getting these mails.  I've had the same phone for a couple of years, so I think it's my phone.  I don't know what has changed but it's very irritating.  I use my email on my phone often for work and need this functionality.