Can e-mail messages be sent from a plus address via Thunderbird SMTP?

Question

Issue

As an example of what I'm referring to:

However, if possible, how to accomplish this isn't evident:

Although this SE WebApps answer cites and expands upon another answer explaining that using https://mail.google.com/mail/u/0/#settings/accounts's "Send mail as" feature, a registered plus address can be utilized, this is a lot of work, rendering this feature more akin to an alias generator than a dynamic and disposable appendment.
Additionally, it doesn't apply to Outlook or Proton Mail, which are services that I utilize more than GMail.

Rationale

The reason I ask for this is because I would like to be able to have the Reply-To header plus-addressed for each Thunderbird installation when sending mail, so that I can trace a message's origin.

I am aware that address obfuscators like https://app.addy.io/aliases exist, but I would still want this feature so that I am able to trace the origin of a message sent via one of those aliases more easily.

Research

I've seen this asked at portal.smartertools.com/community:

Is there a way to send via SMTP from a "Plus Address"?

However, it has been asked nowhere else, and it wasn't ever answered there either. Noteworthily, the question mentions that this capability is supposedly supported if using POP3 (rather than IMAP):

While I can use a plus address to authenticate via POP, it doesn't seem able to authenticate via SMTP.

To a layman like myself, this appears feasible due to how it operates in a more separated manner from the server. However, it doesn't elaborate.

Analysis

If this seems like an X/Y problem, I'll be glad to know in the comments (or even answers) of any relevant solutions, but since I already use AnonAddy and plus addresses where I can, I am confident that I am reasonably familiar with the options available. The focus of this question is about sending mail, not receiving it.

score 2 · Accepted Answer · edited Aug 15 '24 at 14:25

In Thunderbird, this is solved not with separate accounts, but with "Identities".

For any existing account (which you'll have to set up with a 'normal' address, without any + sign), go to:

Edit
Account Settings
Scroll to the bottom
Click on the button Manage Identities
In the popup which opens, click on Add...

Now, insert your email address with the plus, and add other details. Hit OK when you're done. Add other Identities as needed.

Now, when composing a message, you'll get a drop-down menu on the From field, from which you can choose the desired identity.

All identities will use the same SMTP authentication as your main account, so you won't get authentication issues. But in the header of the email you are sending, the From field will contain the plus address.

Consequently, if your e-mail provider accepts plus addresses, this will work.

Additionally, when replying or forwarding an email which has been sent to your plus address, thunderbird will automatically select the corresponding identity in the Fromfield.

Edits

About the Reply-to field:

Thunderbird lets you manually add a reply-to field when composing a message. However, when following the above steps, this won't be necessary since by default the reply-to is the same as the sender, so people who hit Reply will automatically reply to the + address.

Bonus

Use Tools -> Message Filters to sort your incoming email into separate folders, depending on the + address you have used for sending. Read more about Filters in the TB online manual.

score -1 · Answer 2 · answered Jul 09 '24 at 13:12

Plus addressing is supported by Gmail and Outlook/Office 365.

That being said, so many email servers for companies will not accept plus addressing anymore because they dont want people to use it for tracking purposes. I actually tried to use plus addressing recently, simply for sorting email. Every website and service I tried to switch to use a plus address would not accept a plus address or mail to a plus address. Your luck may vary.