Not necessarily. Mail can be intercepted when transferred between MTAs which seem to be often vulnerable to STARTTLS stripping due to a fallback mechanism. See

Not necessarily. Mail can be intercepted when transferred between MTAs which seem to be often vulnerable to STARTTLS stripping due to a fallback mechanism. See "Neither Snow Nor Rain Nor MITM... An Empirical Analysis of Email Delivery Security"[1]. [1]: https://conferences.sigcomm.org/imc/2015/papers/p27.pdf https://conferences.sigcomm.org/imc/2015/papers/p27.pdf