User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Firefox/78.0

Steps to reproduce:

1. Receive a message with a To subaddress like user+subaddress@domain.tld.
2. Click reply.

Actual results:

The From address in the Message Compose Window is missing the subaddress.

This behavior breaks most webmail and ticket systems.

Expected results:

The reply From address should be the same as the message To address.

RFC 5233 states "Implementations MUST make sure that the encoding method used for detailed addresses matches that which is used and/or allowed by the encompassing mail system"

What you want is bug 18882 ?

No, Bug 18882 is requesting an enhancement to support subaddress identities for sending mail, identity detection, identity specific folders, and other complex and likely unnecessary features, but this bug is a defect for the simple problem that a message received at a subaddress should be replied from the same subaddress.

Currently, replying to a message received at user+236745@domin.tld requires manually editing the From address in the Message Compose Window to match the address that the message was received at because "+236745" is improperly being stripped from the From address.

The fix for this defect is simply to stop stripping the subaddress when composing a message.

thanks

digging deeper ... bug 494857