# Proposal for mandating lowercasing when processing e-mail address localparts

[RFC822](https://tools.ietf.org/html/rfc822#section-3.4.7) mandates that
localparts in e-mail addresses must be processed with the original case
preserved. [The Matrix spec](https://matrix.org/docs/spec/appendices#pid-types)
doesn't mandate anything about processing e-mail addresses, other than the fact
that the domain part must be converted to lowercase, as domain names are case
insensitive.

On the other hand, most major e-mail providers nowadays process the localparts
of e-mail addresses as case insensitive. Therefore, most users expect localparts
to be treated case insensitively, and get confused when it's not. Some users,
for example, get confused over the fact that registering a 3PID association for
`john.doe@example.com` doesn't mean that the association is valid for
`John.Doe@example.com`, and don't expect to be expected to remember the exact
case they used to initially register the association (and sometimes get locked
out of their account because of that). So far we've seen that confusion occur
and lead to troubles of various degrees over several deployments of Synapse and
Sydent.

## Proposal

This proposal suggests changing the specification of the e-mail 3PID type in
[the Matrix spec appendices](https://matrix.org/docs/spec/appendices#pid-types)
to mandate that any e-mail address must be entirely converted to lowercase
before any processing, instead of only its domain.

## Other considered solutions

A first look at this issue concluded that there was no need to add such a
mention to the spec, and that it can be considered as an implementation detail.
However, [MSC2134](https://github.com/matrix-org/matrix-doc/pull/2134) changes
this: because hashing functions are case sensitive, we need both clients and
identity servers to follow the same policy regarding case sensitivity.

## Tradeoffs

Implementing this MSC in identity servers would require the databases of
existing identity servers to be updated in a large part to convert the email
addresses of existing associations to lowercase, in order to avoid conflicts.
However, most of this update can usually be done by a single database query (or
a background job running at startup), so the UX improvement outweights this
trouble.

## Potential issues

Some users might already have two different accounts associated with the same
e-mail address but with different cases. This appears to happen in a small
number of cases, however, and can be dealt by the identity server's maintainer.

For example, with Sydent, the process of dealing with such cases could look
like:

1. list all MXIDs associated with a variant of the email address, and the
   timestamp of that association
2. delete all associations except for the most recent one [0]
3. inform the user of the deletion by sending them an email notice to the email
   address

## Footnotes

[0]: This is specific to Sydent because of a bug it has where v1 lookups are
already processed case insensitively, which means it will return the most recent
association for any case of the given email address, therefore keeping only this
association won't change the result of v1 lookups.