The email signature from our customers contains a ‘mailto’.
This looks like <a href="mailto:%20some.name@domain.de"><span lang=EN-US style='color:blue'>some.name@domain.de</span></a>.
This commit lets me believe that Zammad removes all mailto links in the emails and instead displays the destination of those mailto links as plain text. This makes sense as mailto links would lead to your default mail program which would be outside of Zammad if it didn’t remove these mailto links.
Just out of curiosity, why does your customer have a space before his email address in the first place?