I've seen some tangental discussions on this, but surprisingly it seems nothing has changed since Quicken moved to emailing invoices directly instead of using the local email client. Essentially, Quicken "spoofs" the sender email but sends through their own servers. When recipients receive the invoices (if they even receive them!), they show the sender as "unauthenticated" and often due to increasingly strict DMARC policies, these messages are either delivered directly to spam folders or even rejected outright.
In 2024 it is imperative that Quicken properly support a solution for users to send DMARC-aligned messages from the product — be it through their own servers or by re-enabling the existing functionality of allowing the user to send via the local email client. Obviously, for people who send using a 3rd party domain they have no control over (e.g. gmail/yahoo/etc) full DMARC alignment won't be possible. But for the presumably large number of small businesses that send invoices from their own domains, setting up proper DKIM and DMARC records is fully possible and would allow this feature to work much more seamlessly.
There are other alternatives — such as sending the email from a quicken.com email address — that would allow full DMARC alignment as well. Ideally, Quicken would allow for a variety of options to support the widest segment of customers currently trying to use this feature.
