This article applies to 123-reg Mailboxes
After changing your mailbox password, you may find that you cannot send email from your iPhone or iPad. This is because you will also need to update the password for the SMTP server.
Enter your settings, and select the Mail, Contacts and Calendars.
Select the email account you wish to edit.
Under the IMAP heading, select your account.
Scroll down to the Out Going Mailserver section, and select you SMTP account.
Select your Primary Server SMTP details. This should be smtp.123-reg.co.uk.
Here you can update your password to match your new one, allowing you to send emails again.