When several addresses reach one mailbox, the address used to receive a conversation may differ from the address intended for the response. Thunderbird can store multiple identities for an account. Treat its initial choice in the composer as something to review, especially for forwarded aliases and mailing lists.

Inspect the draft before adding recipients or attachments

Open a draft for the intended conversation and expand the From selection. Read the full address, not only the display name. Thunderbird's list can include identities from other configured accounts, so a familiar business name is not enough to identify the mailbox route.

For a new message, Thunderbird normally starts with the selected account's default identity. For replies and forwards it examines the original recipients and chooses a matching identity where available. Mailing-list delivery or other missing recipient information can make that selection less obvious.

Compare the intended From address with the account's existing identity settings. Those settings can also carry signatures and message-copy preferences. If the address is absent, ask the mailbox administrator whether it is an authorised sending identity before adding it. A local entry does not by itself grant permission at the provider.

Check the whole draft after a change

Choose the correct existing identity and reread the draft. Verify the recipients, signature, quoted conversation and any reply-specific wording. An identity change should not become a reason to assume that every other field is now correct.

For a repeatable office check, keep a small note describing which business function uses which approved address. Do not include mailbox passwords or private correspondence. Review the rule with the person who owns the conversation when the correct identity is ambiguous.

Save the draft for the normal content review. Sending is a separate step after the message and recipient scope are approved. The result of this check is a draft with the intended identity visibly selected; delivery and provider acceptance have not been tested merely by opening the composer.

Source: Thunderbird documentation.