Email providers, especially those run by anonymous people (as most .onion email providers are), can go offline at any time. This happened a lot in the past and will happen in the future too. So make sure you always back up the emails you want to keep and do not have important accounts tied to these email addresses
In order to use email securely to communicate you have to pay attention to the following points:
- Choose an email provider from the wiki email section on the main page or a similar one that allows Tor users and is known for not being very responsive to government requests.
- Never give away information in the subject field. For example do not use "about the $4k drug deal we made" as a subject but rather something like "subject".
- Do not forget to check out /r/emailprivacy too. There are a lot of useful tips, guides and links on there.
- Surveillance Self-Defense [ Clearnet link! ] - Tips, Tools and How-tos for Safer Online Communications. A Project of the Electronic Frontier Foundation