domain substitution in email address

I copied an email address from a web site and inserted it into the to: field. The upper case part was set to lowercase. Then I replaced the (at) by the @. Then I wrote the email and sent it. I got a return message, that the receiver did not exist. So I investigated into the source text and found that the domain of the email address had been replaced by some cryptic characters in the souce code but not in the email shown.
How can that happen? Does Emclient or other software replace parts of the text written? That is suspicious! 

P.S.: The address contained a character u+2013,
         see HSU–HH.DE at


There is nothing suspicious going on
The “code”  u+2013 is a  Unicode representation  for the - (dash) in the email address.
It depends on the character set (Font) used.
It also depends which encoding is used for sending - ASCII (plain text) or Unicode UTF8
 HTML  ,but can’t find the setting for this in eMC 
You can choose plain text or HTML, but not encoding,

Unicode Character ‘EN DASH’ (U+2013)

EN DASH (U+2013) Font Support

BTW - For an email address it doesn’t matter if it’s written in UPPER or lower case.