Arnaud De Jonghe I did try it with CTRL + SHIFT + V and I did recreate the E-Mail from scratch, still the huge space at the end appears.
Now Í am sure that it has something to do with my dynamic content since I used a default template, everything appeared fine until I inserted my dynamic content.
In the outlook client it looks like this:

The dynamic content is:
{{#if (eq contact.gendercode 1)}}
{{#if (eq contact.firstname '')}}
Dear Sir {{contact.lastname}},
{{else}}
Dear Sir {{contact.firstname}} {{contact.lastname}},
{{/if}}
{{else if (eq contact.gendercode 2)}}
{{#if (eq contact.firstname '')}}
Dear Madam {{contact.lastname}},
{{else}}
Dear Madam {{contact.firstname}} {{contact.lastname}},
{{/if}}
{{else}}
Dear Sir/Dear Madam
{{/if}}