Email problem after site move

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I'm currently running a site with v5.5.2.1, I have just copied it to another domain on the same server. The copy went fine and the site is fully working and accessible from the new domain name.

There is however a problem with sending email from the standard form. It works fine if the External SMTP settings are left as they were for the email account on the original domain, however when I change the settings to those of a new email account associated with the new domain, email sending fails. Email sending using the new account's SMTP setting works fine outside of C5, I've tested it with my email client.

I'm guessing there is some configuration change I need to make?