For most Listserv lists, there is a security feature where list editors will need to approve a message with a confirmation link that gets emailed back to the sender after the initial message is sent. With most email clients, this feature works well. For users of Outlook 365 / Outlook online, there is an additional security feature that they offer called Link Preview which conflicts with the Listserv's confirmation security feature.
Link Preview automatically opens up all emails to links in your emails. The reason for this feature is in case there are any malicious links, Outlook will catch them before you click on them. The downside is that when you have a link such as the Listserv confirmation, the Link Preview feature causes the confirmation to be accepted automatically.
Here is more information about this feature and how to disable it:
Because of the known benefit of such as feature, we cannot endorse disabling this feature without first expressing the potential consequences that such a change may open up for unsuspecting users who may be prone to clicking on malicious links in emails. Always proceed with caution!