How can I search for meeting invites in Microsoft Outlook for Mac
The "Raw Query" search option can be used for this purpose.  Just search use the following raw query:

com_microsoft_outlook_has_reminder == 1




From what I have been able to determine, the raw query field uses the Spotlight file metadata query expression syntax.  I haven't found an authoritative source that states this, but various online posts and manual testing confirms this to be the case.

As mentioned in this gist, it looks like the list of raw query options can be found in the following file within the Microsoft Outlook app:

/Applications/Microsoft Outlook.app/Contents/Library/Spotlight/Microsoft Outlook Spotlight Importer.mdimporter/Contents/Resources/schema.xml

Based on the name and list of attributes, it looks like the com.microsoft.outlook15.message type is the one that applies to email messages.

&lt;type name="com.microsoft.outlook15.message"&gt;
    &lt;allattrs&gt;
        kMDItemTitle
        com_microsoft_outlook_recordID
        kMDItemContentCreationDate
        com_microsoft_outlook_messageSent
        com_microsoft_outlook_messageReceived
        com_microsoft_outlook_priority
        com_microsoft_outlook_flag
        com_microsoft_outlook_unread
        kMDItemContentModificationDate
        kMDItemCoverage
        kMDItemKeywords
        kMDItemProjects
        com_microsoft_outlook_has_text_content
        kMDItemTextContent
        kMDItemAuthors
        kMDItemRecipients
        com_microsoft_outlook_folderID
        com_microsoft_outlook_junkLikelihood
        com_microsoft_outlook_size
        com_microsoft_outlook_newsAccountID
        com_microsoft_outlook_accountID
        com_microsoft_outlook_repliedTo
        com_microsoft_outlook_forwarded
        com_microsoft_outlook_redirected
        com_microsoft_outlook_toRecipients
        com_microsoft_outlook_toEmailAddresses
        com_microsoft_outlook_ccRecipients
        com_microsoft_outlook_ccEmailAddresses
        com_microsoft_outlook_bccRecipients
        com_microsoft_outlook_bccEmailAddresses
        com_microsoft_outlook_attachments
        com_microsoft_outlook_has_attachments
        com_microsoft_outlook_has_mentioned_me
        com_microsoft_outlook_projects
        com_microsoft_outlook_categories
        com_microsoft_outlook_primaryCategory
        com_microsoft_outlook_flagged
        com_microsoft_outlook_author_email_addresses            
        com_microsoft_outlook_recpient_email_addresses
        com_microsoft_outlook_isFromMailingList

        com_microsoft_outlook_hasStartDate
        com_microsoft_outlook_startDate
        com_microsoft_outlook_hasDueDate
        kMDItemDueDate
        com_microsoft_outlook_has_reminder
        com_microsoft_outlook_completed
        com_microsoft_outlook_completedDate

    &lt;/allattrs&gt;
    &lt;displayattrs&gt;
        kMDItemContentCreationDate
        kMDItemAuthors
        kMDItemRecipients
        kMDItemCoverage
        kMDItemContentModificationDate
    &lt;/displayattrs&gt;
&lt;/type&gt;