Invitations from Huddle are sent out from firstname.lastname@example.org on behalf of your email address. Some email servers will block these emails because they appear to be 'spoofing' the organisations address due to them being 'on behalf of username'. This is however standard & industry accepted web protocol for sending emails and is not actually 'spoofing'. This format of email also provides a benefit to Huddle users because if someone replies directly then the email will go to the user and not back to Huddle.
We understand that some mail servers have some stricter security policies. Adjusting your firewall settings will not enable these invitation emails to come through. You will need to get your system mail administrator to allow these 'on behalf of' emails by adding 'email@example.com on behalf of' as a 'safe sender'. It is likely that these emails will be blocked at source rather than by a junk mail filter.
By default, all Huddle user accounts are set to receive email notifications. In the event a user is still not receiving those notifications, it is recommended they verify they have not chosen to exempt themselves from those notifications. You can find this setting by going to your profile icon in the upper right hand corner of Huddle and clicking "My profile." Select "Edit your profile," scroll down to "Send notifications by email:" and make sure the box is checked.