I just received reports from different customers having issues when sending e-mails after upgrading to iOS7. It happens with all e-mail providers, Zimbra too. When it doesn't work, the symptoms can be one of the following :

- The connection to the SMTP server fails
- The e-mails are blocked in the outgoing queue and are sent multiple times

For some people the problem comes and goes. Some report that disabling the WiFi fixes the problem (!?). Other have reported success after removing the account, reboot, create the account, but then in blocks again a few hours later.

In short, it smells like a bug in iOS7. If you have not upgraded yet, wait for a while ...