Mail alert service only available in some projects

I have two projects belonging to the same team. When I configure alerts for project A, the “Mail” notification service is available and I receive emails.

For project B, no Mail service appears in the list of alert options and emails are not sent:
The Slack notifications work as expected though.

I can find no differences between the project settings. Has anyone else experienced this?

This is Sentry 8.15.0 (Python install without Docker). Tempted to upgrade just to see if it helps, but I’d like to avoid that for now.