[Exim] spaces and Eudora

I have received as Postmaster one messagese , misrouted, that had as
To address "%20realusername@???"
The sender used Eudura Pro 3.2 windows.
It looks that the addresse was url-escaped.
RFC822 state that spaces in address are not significant, so either
" realusername@" and "realusername@" should be considered the same.
However this occurred . Where is the fault (apart on the human sender)