handle receivers that contain '/'

[siretart]

Fixes: #23

[Timoses]

I would add this as an example in the .env.default so it's clear. Perhaps even document it in the above comment so it's clear?

Actually, the comment above could document the rest a bit better. Now that I look at it (haven't done so in a while), I don't remember what the !abcdefgh: in receiver1/!abcdefgh:homeserver.tld stands for.

[siretart]

done, PTAL