SMTP mail domino -> sendmail?

we have a domain company.co.ukour domino smtp host is smtp.company.co.uk

we have unix servers eg [servername].company.co.uk that run sendmail. These servers email domino users fine via SMTP via the domino smtp server. However, emails from users to eg [mailbox]@[servername].company.co.uk bounce with message “User [mailbox] ([mailbox]@[servername].company.co.uk) not listed in Domino Directory”

How can I sort this problem please?