XMPP -> Matrix doesn't work when the MXID has capital letter

[Mikaela]

Describe the bug

Matrix users whose MXID contains a capital letter (from time before that became disallowed) cannot be messaged from XMPP as according to Maranda:

You can't have caps in a JID that's against the prep profile so XMPP servers will normalize that

To Reproduce

1. Send a message from XMPP to a Matrix user e.g. [email protected]
2. The message never arrives.

Expected behavior

The messages go through and I can read them at Matrix.

[maranda]

Just for clarity: "you can't have caps in the node portion of a JID"

[Mikaela]

I guess this is an instance of matrix-org/matrix-spec-proposals#3794

[JokerGermany]

This is related to #281

[Half-Shot]

We'll probably have to solve this by QP encoding localparts (which will be ugly but functional).

[Half-Shot]

This is a S-Minor because it technically affects only users who are not spec compliant.