This will depend on the service that is blocking the email.
If the email is showing up in your UWSPAM Quarantine digest, you can approve the message and the sender through the UW SPAM Blocking site, which will whitelist the sender for future mailings.
If the email is showing up in your Office 365 email from quarantine@messaging.microsoft.com through you can whitelist it for your account by adding it to your safe senders list.
If the message is blocked at the gateway by MTA level IP blocking, send an email to UserHelp@uwyo.edu with the sender's email address and the reason that the sender should be whitelisted. If possible, also provide the notice received by the sender that indicates the message was blocked.
If the message is being filtered into the Junk email folder by a rule you have created in Microsoft Outlook, you can create an Exception Rule to whitelist this sender. See Rediect your UW Email in Outlook for assistance.
If you are using the Junk email filter in your email application, and the message is being filtered into the Junk email folder, add the sender's address to the Safe Senders list.