You tested a campaign to Outlook and you notice that your hyperlinks do not show the tooltip/alt text set when designing the email.
Unfortunately, there is no foolproof answer to making Outlook render alt text.
This issue is not solely an Outlook issue, other email clients like Mac Mail appear to have the same rule regarding hyperlink alt text - not displaying the alt text but instead displaying the actual URL.
It appears the thinking behind displaying the URL itself instead of the alt text is a security precaution -- that recipients would want to know the actual URL they are about to click on in case of malicious intent.
Outlook and other email clients also override the alt text used for images.