I am using iOS 6.1.4 on iPhone 5, and I've noticed that at some point in the past few weeks - I do not know exactly when it began - any email I try to send from within any non-Mail app, such as sharing links from within Safari, InstaPaper, Twitter, or Tweetbot, are never received by the recipient. The email does not show up in my Sent Items folder either. There is no error message displayed when trying to send these emails - I press Send and the email window is animated into the background as normal.
I do not have any problem with sending emails from directly within the Mail app itself, these are received fine by recipients and show up in the Sent folder as expected. I have verified that the emails that originate from within Safari or other apps are not simply ending up in a Spam folder.
I have two email accounts set up on my phone, Gmail and an Exchange account for work that I leave disabled/inactive most of the time. The Gmail account is set up as Exchange and the server name is listed as m.google.com with Use SSL enabled.
Has anyone seen anything like this before or know what might be the cause?