are services like free-email-forwarding safe ?, can the snoop into and see my emails?

basically can services like https://github.com/forwardemail/free-email-forwarding snoop into my emails, despite being open source, that does not mean the source code and the server and and client end is going to match, So can these services read my emails ? Are there further resources I can read to feel more comfortable using these services ?

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