Host mastodon servers
Mastodon federated micro blogging servers are receiving massive influx of twitter refugees. To create a new server, people need both Linux sysadmin knowledge, and also ability to moderate content and build a community of users. Proton could offer to host new mastodon servers and do the sysadmin jobs, including backups and installing updates. The remaining tasks, especially content moderation, will fall to the mastodon operator.

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Jake commented
Proton should host a Mastodon instance for its paid members to join (e.g. @johnsmith@proton.ch).
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Christian commented
I'd be happy with a heavily restricted/single user mastodon/activity pub instance. The primary reason I use proton mail is to control my identity (custom domain). A single-user mastodon instance would be a perfect extension of that.