SharePoint external sharing - external users not receiving email invite?

I've been experiencing a huge amount of difficulty in trying to enable external users access to one of our SharePoint sites. We have a site collection, called 'Projects', and I am attempting to share one of its subsites with a couple of people in another business.

Inside the SharePoint Admin Centre, I've ensured 'New and Existing Users' is enabled in the sharing settings. In addition, this setting is also enabled for the site collection itself. I have tried and tested sharing the site externally with my own personal @gmail.com account and it works fine.

When attempting to share the site with other domains, such as @hotmail.co.uk or @uk.gt.com, the users are receiving nothing to their inboxes (or spam folders). When I share the site I receive no error message in SharePoint, and the user's name does not appear in the 'invitations and requests' area under guest users.

It's also worth mentioning I haven't included any message in the invite, as I know the invitations can have a 256 character limit.

I'd be extremely grateful if anyone can help with this as numerous people are depending on it to access materials held on the site and I cannot find any issues with the setup!