To allow Squarespace to send emails on your behalf, you will have to add them to your SPF record. The SPF mechanism used by Squarespace is shown below.

include:squarespace-mail.com
​

For DKIM support, you need to create a CNAME record in your DNS.
It should have the following properties:

Host: squarespace._domainkey
Value: squarespace-domainkey.squarespace-mail.com
​
For more information please ckick the button below.

Create a free OnDMARC account to test your configuration.