Please reconsider recycling usernames.
Providers like Fastmail or Mailbox.org do it and on those services users can't even keep an account active without paying forever.
Tutanota offers free accounts so anyone who wants the username not to be released could simply keep the account instead of deleting it. Many high value usernames have been wasted because they were deleted. It is still early days for your company in the big scheme of things so it isn't too late to make change.
I believe recycling usernames is an uncontroversial choice because
1. You provide free accounts.
2. You don't request any personal information.
I know what your answer is most likely to be but I still wanted to offer this perspective.
From a very happy paying customer
Before deletion you must warn users that the email address will be recycled and any accounts that they have made that are still active will be accessible by anyone who reuses the email.
@anon November 15- OP didn't suggest accounts to be deleted after inactivity. Only usernames to be recycled when the user deletes the account themselves.
As the next happy paying customer I think the Tutanota policy is very correct, for the by them mentioned reasons, and a reason for me to switch from free to paying account. If you like the system so much at Fastmail or ... you're free to go overthere :-)
@anon Typically how this happens is, that after 6 months of inactivity, an e-mail is deleted (current practice here also). After 12 more months of quarantine, the e-mail is then released to be reused.
This could easily go wrong, however. Imagine John A wanted email@example.com, and so he took that email. But he forgets to sign in one month. Then, John B comes along and takes the recycled domain firstname.lastname@example.org.
The problem now is that someone could email email@example.com something intended for John A, when in reality John B receives it. This could be worrying, especially if the contents are private (which is likely to be, given the nature of tutanota).
This is basic housekeeping and needs to be done. Tutanota should not be used as a parking lot for dead email accounts. It's great that there's a free option but it shouldn't be free forever simply because you can't be bothered to code a housekeeping routine to run once every 2, 3, 6, or 12 months to prune the dead email addresses.