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. Please follow the below steps to enable SMTP authentication.
Step 1: From the Home screen click "Settings" and then "Mail, Contacts, Calendars"
Step 2: Select the email account you wanted to modify.
Step 3: Scroll down to "Outgoing Mail Server" and click "SMTP".
Step 4: Click on the "Primary Server" and make sure the slider is "ON"
Step 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 ON
Step 6: In the "Authentication" section set it to "Password"
Step 7: Set the "Server Port" to 465
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.
Step 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.