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The answer to all of these is no.
But the following form strongly suggests at the bottom that email is acceptable. http://hmctsformfinder.justice.gov.uk/courtfinder/forms/n215-eng.pdf
I have been in a year long correspondance with her daughter about all manner of issues to do with the flat on the same email address.
Specifically regarding the s21 notice, she did not reply to the email in which i served it.