Retry SMTP login on transient failure

[link2xt]

SMTP login sometimes fails with

src/smtp.rs:145: SMTP failed to connect and authenticate: SMTP failed to login: transient: 4.7.0 Temporary authentication failure: Connection lost to authentication server.

In this case we should probably try again after 30 seconds or so. This makes configuration fail sometimes, e.g. tests/test_something.py::test_configure_alternative_port fails in CI due to high load CI puts on chatmail server authentication.

We can also improve performance of chatmail authentication to reduce this problem, but temporary authentication failure may be a problem when configuring non-chatmail accounts as well.