reply on email destinated to a subaddress of an identity goes in not found and defaults to initial identity

Checklist

• I have used the search function to see if someone else has already submitted the same bug report.
• I will describe the problem with as much detail as possible.

App

Thunderbird for Android

App version

9.0

Where did you get the app from?

Google Play

Android version

14

Device model

Samsung S21 FE

Steps to reproduce

setup an alias on your email server such as alias@domain -> user@domain
add alias to identities
write an email to a subaddress of that alias such as alias+abc@domain
click reply or reply to all

Expected behavior

fill the from field with alias identity[alias@domain], with "reply to" field filled with subaddress [alias+abc@domain]

or fill from with the subaddress direcly

I’ve made a small, amateur modification for myself, and I’ve been using it daily for a while. I’m not sure if it meets your code quality standards, I think we could consider it a proof of concept
it also add the support to custom RECIPIENT DELIMITERS different from the standard +.

Actual behavior

it does not match alias[RECIPIENT_DELIMITER][any]@domain as equal to alias@domain and fallback to initial identity [user@domain]in from field

Logs

No response

Start with the listed alias and subaddress reproduction steps in Thunderbird for Android 9.0, then compare the current behavior with the linked proof-of-concept branch. Trace the reply identity matching entry point and verify that replies use the alias identity with the subaddress in Reply-To, including custom recipient delimiters.