To authorize Namecheap Email Forwarding to send emails on your behalf you will have to include it in your SPF record. The SPF record mechanism used is shown below.

include:spf.efwd.registrar-servers.com

For more information on SPF please click on the button below.

For information on how to enable DKIM for your email forwarding service or if it is even necessary please contact Namecheap's support team. 

Create a free OnDMARC account to test your configuration.