Hello @JenHenczel ,
1) To get started with everything authentication-wise, I'd advise looking over our main article on self-authenticating (also available via the Help tab in accounts).
For more specific info regarding how DMARC works, I'd recommend these articles:
What is a DMARC policy and why do I need one?
DMARC reporting errors with self-authentication
I'd also advise checking out our webinar on the topic from last week. We'll be updating the post with other questions and answers, as well as other highlights as time goes on.
2) I'm not sure I really understand this question. Emails can still be forwarded, and if you have a particular address set up for reply-to's, those are what will be utilized by email programs' Reply functionalities if a contact selects that.
3-4) The various top-level domains (.com, .org, .net, etc.) aren't necessarily that important, but it may determine the availability of some domains, as well as denote what your organization is. For example, an art page might prefer to have a .art TLD, whereas an agency that connects and manages multiple businesses, franchises, or other systems might have a .net TLD. This is completely up to you if you want something more standard and generic, or something more unique to stand out (assuming it's available to purchase from your domain host).
5) This will depend on where you've purchased your domain, as some site builders offer this capability by default, and others require using an external system like Workspace to set it up.
See also:
Email Authentication FAQ
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William A
Community & Social Media Support