The current method of joining a team is to accept an invite email... but several of the major email service providers (e.g. all Microsoft based servers) don't like this invitation email, so the emails are not delivered.

It would be good if there were an alternative way to join someone's team when these emails are not being delivered. Perhaps if the join link can be copied by the person inviting you to their team and then sent from their email address directly (e.g. "copy this link and send it to the person you're inviting")?