HTML mail is sent as text/plain

My mail client (OSX Mail) shows the HTML tags contained in the confirmation mail as plain text:

Hi mr. x,<br/><br/>please click the following link to proceed with the sign-up process.<br/><a href="https://domain.com/sign-up/confirm/e65b5023-4249-4181-a388-1964f0a9d841">Kiezatlas Nutzerzugang best&auml;tigen</a><br/><br/>You'll receive another mail once your account is activated by an administrator. This may need 1 or 2 days.<br/><br/>Cheers!

https://domain.com/

I guess this is because text/plain is set as content-type (instead text/html):

------=_Part_18_2102533876.1501317748038
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit

Hi mr. x ...
------=_Part_18_2102533876.1501317748038--

Strange also are the <br/> (instead of <br>) tags, which is XHTML (the proper content-type would be application/xhtml+xml), not HTML.