We're aware that the US webmail server has been having intermittent timeout issues, particularly during US business hours.
We're working to resolve this but we expect that it will take at least a couple of weeks. Our recommended workaround in the interim is to use a desktop mail client or to run your own webmail server separately from our managed webmail instance.
Desktop email client configuration info is available in our user guide at: Email Client Configuration
If you'd like to run your own webmail server setup instructions are available at: HOWTO run your own Roundcube webmail server on Opalstack