If you're able to receive email on your iPhone / iPad / iPod touch but cannot send email from it, most likely you need to enable or re-enable SMTP Authentication on your device, and this solution will work in almost all cases.
Before you begin the steps below it's important to note that there is a known strange behavior with some iPhone / iPad / iPod touch devices - when you change the settings for the Outgoing server, your device may give you an error like "The server could not be verified" or "Cannot Verify Server Identity", and you have to just click "Continue" and sometimes the error will occur 3 or 4 times before it finally goes through. (This is "normal" behavior according to the Apple Support community). So, keep that in mind as you'll likely encounter it at some point during the steps below.
How to get your iPhone, iPad, or iPod Touch to Authenticate SMTP (the Outgoing server) so you can send email:
1. From the Home screen click "Settings" and then "Mail, Contacts, Calendars"
2. Select the email account you wanted to modify.
3. Scroll down to "Outgoing Mail Server" and click "SMTP".
4. Click on the "Primary Server" and make sure the slider is "ON"
5. Enter the host name in this format:
But replace the "example.com" part with your domain name and then:
Enter your username (your FULL email address) and the password you assigned to that account
Change "Use SSL" to OFF
6. In the "Authentication" section set it to "Password"
7. Set the "Server Port" to 587
Note - as mentioned toward the top of this article, when you change these server settings may encounter a "Cannot Verify Server Identity" message, sometimes several times in a row, and you may have to click "Continue" each time to move forward.
Depending on your device, you may need to click a "Done" button now.
8. Go back to the main "Settings" page and then get out of the config all-together.
You should now be able to send email from your device since it is now properly SMTP Authenticating.