There is currently an issue with the Webmail interface at https://webmail.webfaction.com which is resulting in sporadic 500 Internal Server Errors for some email users. We are looking into the problem and hope to have normal service restored soon. We will update this entry as soon as we have more information.
2018-03-02 14:50 UTC: The root cause of the errors has been fixed. Thank you for your patience, and if there are any issues please feel free to contact support by opening a ticket.