Address book email addresses limited to 3 per contact in Webmail UI

Mailbox.org's Webmail UI seems to support only 3 email addresses, despite other devices synced through CardDAV supporting additional email addresses.

Additional email addresses beyond 3 shown on another device (such as Thunderbird or Android Contacts), and synced via CardDAV, don't appear in the Webmail UI.

The good news is that, in a test, a 4th email address added via Thunderbird *did* sync via CardDAV to Android Contacts. The 4th email address simply never appears in the Webmail UI, without warning to the user. But at least the other devices can sync it.

This seems to be due to a limitation of the design of the Webmail UI. It seems to support only 3 fixed email addresses (English labeling "Email 1", "Email 2", "Email 3").

I do not know whether there is some pattern of changes to a contact with more than 3 email addresses, which could cause worse problems or data loss during synchronization.

The applicable Internet standards themselves (IETF RFC 6352, 2426) seem to support an arbitrary number of email addresses, and modern devices seem to also support this.

Perhaps the Webmail UI could be improved to also support arbitrary numbers of email addresses per Contact (as well as the role labels for those email addresses)?