If you are using secondary domains for creating sending accounts, it's recommended to set up forwarding to your main domain (website). All users that request an URL of your new domain will be automatically sent to your primary domain or your landing page. Watch the detailed video by clicking the link here.
Here are the steps to set up your forwarding.
Log in to your Namecheap account.
Navigate on Domain List, find your domain and click Manage.
In Domains tab, scroll down to Redirect section.
Click Add Redirect.
In Source URL, type your secondary domain and in Destination URL, type your primary domain. Remember to include http:// or https:// with your primary domain.
Click on Check mark to save changes.
Now you are all set. When clients who receive your email try to go to your domain from your sending account, they will be redirected to your main site!