The reason for this is email authentication, which is a way for email recipients to verify that an email was sent by who it claims to be from. Email clients like Outlook and Gmail then display this extra information to help recipients determine which emails are legitimate.
For example, if you send an email from [email protected]
we will authenticate your emails with our domain instead, and sign your email using our domain key. This has a direct and positive result on your delivery rates, and has no negative side effects.
This means some email clients will then display our domains — such as cmail.com or createsend1.com — as extra sender details in the "From" field, as shown in the example below: