It's all well and good to run your own mail server. The biggest problem is 99% of who you send to/receive from will be in one of the big 3 mail servers.
If the threat model for the communication within your group is concerned with attack or coercion from hostile governments, then you have the ability to set up your own email server and convince your participants to switch to it. It's not really much to ask -- it's fairly routine for companies and organizations to issue email accounts to employees/members and require their use for official business.
Consider that it may be easier to ask your communication partners to start using a new email account you've provided them than to adopt a new communication platform.
On the other hand, the people you're communicating with have chosen to trust the organizations that they're using to provide them with those services. Whatever service they're using is not radically different to trust than it is to trust the Signal developers, or the Google Play Store or Apple App Store to distribute the Signal application to your mobile device, and so on. Even if your participants are using Signal to communicate, they may have online backups enabled with their device to Apple, Google, etc., such that without your awareness your communication partners are trusting them just as much as if they were email providers.
I just had a quick look at a email list I run for a local sporting organization and it looks like the big 3 are less than 50% of addresses. There were a lot of local ISP addresses, businesses and educational organizations.
2 things to consider. A lot of people use more disposable accounts for mailing lists. And a lot of businesses and educational orgs use Google apps or MS's equivalent with their own domain name.
Are you basing the 50% based on what is after the "@", i.e. gmail.com and yahoo.com? Or are you looking at what server the MX points at. It's very easy to have your own MX for your domain point to a google mail server.
If the threat model for the communication within your group is concerned with attack or coercion from hostile governments, then you have the ability to set up your own email server and convince your participants to switch to it. It's not really much to ask -- it's fairly routine for companies and organizations to issue email accounts to employees/members and require their use for official business.
Consider that it may be easier to ask your communication partners to start using a new email account you've provided them than to adopt a new communication platform.
On the other hand, the people you're communicating with have chosen to trust the organizations that they're using to provide them with those services. Whatever service they're using is not radically different to trust than it is to trust the Signal developers, or the Google Play Store or Apple App Store to distribute the Signal application to your mobile device, and so on. Even if your participants are using Signal to communicate, they may have online backups enabled with their device to Apple, Google, etc., such that without your awareness your communication partners are trusting them just as much as if they were email providers.