How masked is a Fastmail "Masked Email"?

[chrisjj]

Specifically, can the originator's ID (e.g. email address) be determined from a FM masked email?

I ask esp. because I recall the answer for the now gone/renamed FM Aliases feature was: Yes - trivially. E.g. send an email that provokes a rejection.

FTR, FM UI says

"
Keep your real email address private. Make a unique email address to use everywhere you log in on the web. Add a description to track who should have it. Block addresses if they start sending spam.

Use the Fastmail app or web interface to reply from a Masked Email address, so your real address remains hidden.
"

Help https://www.fastmail.help/hc/en-us/a...1-Masked-Email
"
A Masked Email address is a unique, automatically generated email address that can be used in place of your real email address.

Masked Email addresses are especially useful when you need to sign up with new services online. Instead of sharing your real email address, keep it private and protect yourself from data breaches and spam by creating a new Masked Email for every service.

If a Masked Email address starts receiving unwanted mail, you can simply disable that address. Masked Email addresses also make it easy to identify which service shared or leaked the address.
"

[hadaso]

Originally Posted by chrisjj

Specifically, can the originator's ID (e.g. email address) be determined from a FM masked email?

I ask esp. because I recall the answer for the now gone/renamed FM Aliases feature was: Yes - trivially. E.g. send an email that provokes a rejection.
...

I sent an email from my Gmail account to an alias in my Fastmail account that bounces mail (bounce@mydomain).
The rejection report showed no email address other than the ones in the mail sent (the alias and the gmail address of the sender).
I don't use the "masked email addresses" feature.

[chrisjj]

Great. An improvement for aliases, then.

You'd hope the same applies to masked.