Using email masking services like SimpleLogin or Proton doesn’t significantly increase the risk of your email contents being leaked, but it does add an extra service that processes your emails, which means one more point you need to trust. In practice, reputable providers use strong security and encryption, so the added risk is small.
If you use something like Proton’s own aliases, your emails can remain end-to-end encrypted, which is very secure. If you use a forwarding service like SimpleLogin to a provider such as Gmail or Outlook, the email is processed and forwarded, so both services could technically access the content, making it slightly less private but not dramatically less secure.
Overall, email masking is usually a net benefit because it protects your real email address, limits damage from breaches, and reduces spam. The biggest security risks still come from things like weak passwords or phishing, not from the masking itself.