This CSS can fix this problem, but it will only work with block elements (p, div, .. etc):
mso-line-height-rule:exactly; line-height:10px;
If you are trying to create a vertical distance, use line-height and font-size to provide the height:
line-height:5px;font-size:5px;height:5px;
Outlook.com (Hotmail) will override your CSS line-height with their help, so you will need to use it to "reset" your CSS after changing it:
.ExternalClass, .ExternalClass p, .ExternalClass span, .ExternalClass font, .ExternalClass td, .ExternalClass div {line-height: 100% !important;}
Outlook.com continued: then if you have any elements with line-height:0 , you will need to specify the id attribute for them, and then specifically reset those:
.ExternalClass #elementWithNoLineHeight { line-height:0 !important; }
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