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Mailing list admin
The tradition of doing most communication in an Open Source project over email and mailing lists is probably a habit that is dying, but as I am an old dinosaur I have always made sure my Open Source projects have mailing lists and associated archives for them so that you can look up mails afterward and link back to them etc.
Someone then needs to make sure those lists are hosted somewhere, that they run properly with as little downtime as possible, that there is a decent amount of spam countermeasures and that the list can properly handle new people signing up or leaving.
I have had to learn a lot about mail servers and mailing list software.
These days, it is popular to rather use other communication means. Forums, chat software, video meetings in different combinations and setups. The point is still the same: someone in your project needs to make sure they are managed, configured, setup and working for the team.
These days there are a lot of online services that provide these things so that you do not have to self-host or run them yourself on your own servers, but they still typically need some amount of hand-holding. As a project maintainer, you can bet that you will be one of the people expected to deal with it.