I suspect that you have not resolved this issue. Outlook is way out of date and well past its best before date.
Specifically OL2007 does not support TLS encryption and SSL has fallen out of favour because of security concerns. The usual encryption for the POP3 server on port 995 is SSL/TLS which allows TLS to be negotiated ... OL2007 does not offer that option.
Have you set encryption on the SMTP server (port 587 with STARTTLS)?
Are you authenticating with the SMTP server (use the same authentication details as the inbound server)?
You have not disclosed that target domain yyy.co.uk - some email providers have become really prissy over security on inbound emails - it is possible that they are rejecting receipt of your email because the submission (a) was not authenticated and / or (b) was not done over an encrypted link.
It would be wise to invest in a maintained version of outlook, or switch to one of another (free ?) email client.