Why are there two separate webmail interfaces, mail.* and webmail.*?
Probably same reason there's GMail Standard & GMail Classic.
The classic more system resources friendly.
Probably? Are we guessing here or do we know?
@pafflick just so you know, you're not actually answering my question. You're telling me things I already know.
@msava The mail* interface was the original implementation.
Then there was an update to use the webmail* interface.
However, rather than disable the "old" one, both were kept available to allow user choice (the Vivaldi ethos).
@msava Well then, thank you for clarifying that (I guess?). Just so you know - your OP did not make that obvious.
As for the question: In the beginning, there was only the "old" Mail interface (based on Roundcube) - some people got familiar with it (and thus got used to it - besides, it's used on other webmail services on the Internet). Then they introduced their own interface, which became the "default" one over time. They left the Roundcube one though so that people can still use it (if they prefer).
EDIT: whoops, look's like @TbGbe was faster by a few seconds... 😉