If you have multiple domains, you can make use of the domain aliasing option. Domain aliases can be added to your main domain directly in the web interface. Any email sent to the domain alias will be delivered to the same user on the main domain.
Example
Let's presume you have the main domain example.com, with destin ation mail server mail.example.com, and the alias domain example.ie. Email sent to myemail@example.ie will be delivered to the SMTP server mail.example.com with the recipient myemail@example.com. The email "To:" headers will still show the origin al recipient myemail@example.ie'.