[Dovecot] Rewriting username at login

[Gerry Demaret]

I am trying to make a switch from Cyrus to Dovecot which has some
"historical accidents" to it that I have to deal with and ideally would
like to get rid of.

One of them is that about half of the existing accounts have a
different format for their login. All the newer accounts have their
email address as login, the old ones use a dot instead of an at:

- new: user1 at domain.tld
- old: user2.domain.tld

I would like to migrate away from the all-dots notation since it leads
to a lot of support questions, and for that I'd like to run in a
compatibility-mode for a few months. Effectively, I want to get rid of
all the old users and convert them all to the new scheme, and have the
username rewritten at login, so every second to last dot gets rewritten
to an at.

This would ensure that we have no complaints and allow our users to
migrate slowly.

All users are currently stored in a MySQL database for Cyrus, but this
will be changed to a Dovecot-LDAP combination.

Is this possible? How could I achieve this?

Kind regards,

Gerry.