To move an email to the spam folder ...

1. Select or open an email
2. Press the 3 dot menu.
3. Select "Spam"

Thunderbird Android doesn't have any filtering or blocking features like the desktop version.

You can find this feature requested by the community on Mozilla connect link below. You can show your interest by up-voting it, or raising your own ideas.

https://connect.mozilla.org/t5/ideas/filtering-with-android-thunderbird/idi-p/61144

unfortunately there is no option to select spam in the dot menu.

Are you using an imap or pop3 connection. The server-side spam folder is only available with imap connections. Pop3 has no folder synchronisation capability.

If you use imap and the spam option is missing ...

1a. Navigate to settings > account > folders 1b. Check the "Spam folder" is assigned. Typically = automatic (spam)

2a. Navigate to settings > general settings > display > visible message actions 2b. Verify the "Spam" checkbox is ticked.

3. If spam is not immediately displayed under the 3dot menu, it may be in the refile sub menu.

Most mail providers offer server-side filtering, configurable from their webmail settings page. This may offer you an alternative solution.