[solardev]

Is this really a question of software vs deliverability setup? Did you validate all the SPF, DKIM, and DMARC stuff already?

If your email group is for internal use only, can you set it up at the level of your mail server instead? It should be able to just copy all the messages to the other users instead of sending it over the internet. eg for Google orgs, https://support.google.com/a/answer/9400082?hl=en

[theallan]

SPF, DKIM, DMARC are all set up correctly. There is no issue with deliverability when just sending from an indiviual account of the charity's domain to Outlook / Yahoo, it is purely when going through Google Groups that there is an issue.