Assuming that everything is done right (everything, which is not a given especially if one does it oneself),
HTTPS still would't protect your email as emails are not using that protocol to be transmitted. Because you actually need to send the actual email to other servers using the "email protocol":
SMTP. But even then, this is a bad idea.
For instance, what are these "other servers" exactly? If you recipient is at gmail for instance, you have to send the email to the gmail servers, right? However this is not exactly how
SMTP is working: you email will hop from mail servers to mail servers until it reaches its final destination. This connection can be direct but there are no guaranty that it will actually be, far from it. This simple fact makes end-to-end encryption within
SMTP impossible by design: each realy/MTA is a MitM! Yes, there were attempts at making
SMTP secure but the design is still just bad from a security standpoint. See this Wiki article for instance, especially:
Mandatory certificate verification is not viable for Internet mail delivery. As a result, most email that is delivered over TLS uses only opportunistic encryption.
Opportunistic here means that it defaults to clear text whenever someone illegitimately wants to read, if anything in the middle is broken, if anything is not configured properly, etc. And the worst is that it's not because your connection to the first relay (called MTA, in the other answer) is actually secure (because you did a good job wasting 8h of you time in configuration and thorough checks) that the connection between that very first relay with the second will be secure. You have no guaranty of that whatsoever (and again, the relays themselves can be evil).
Don't send clear-text emails. Use PGP/GPG, S/MIME or equivalent to crypt your emails if you want secrecy. These provide end-to-end encryption but require a fair bit of configuration from your side and from your user side. To be done properly (and actually provide privacy), you need to manually authenticate each user. This may not be possible.
Another solution would be to use a protocol already designed to be secure which provides end-to-end encryption (some IM protocols are actually quite good for this).
As proposed in the other answer, provide an email with a link to a protected
HTTPS page only accessible to authenticated users (a token in the URL won't do).
However, never ever use
SMTP (with or without TLS/StartTLS) if you need a guaranty of a minimal amount of privacy.
SMTP is not secure.