I have a user whose 2FA never arrives via email. As a result, they are unable to login to Atlassian products.
Their email was recently reconfigured by IT, but the address is the same as what is listed in the Atlassian Administration User directory. We do not have Atlassian Guard. While their email does function, it also appears they are not receiving any notifications from e.g. Atlassian Confluence at their email. Thus, this suggests the problem may be on the Atlassian side.
A possible solution is to disable 2FA for this user, but I do not see a way to do this from the Administration page.