How do I block unwanted emails in Thunderbird?

It depends on what you mean by “blocking unwanted emails.”

If you mean spam, Thunderbird has a built-in spam filter, just like Outlook
Express had. While no spam filter is perfect, marking something as spam is
pretty easy.

If you mean that you want to block specific email addresses from showing up
in your inbox, that too is very similar to Outlook Express’s approach. Just be
aware that blocking senders by email address is not an effective way
to stop spam, no matter what email program you use.

Thunderbird’s Junk Mail filter

Like some email programs, Thunderbird refers to spam as “junk mail” and
includes a junk mail filter that can be trained over time – the more email you
marks as spam, the better Thunderbird gets at identifying spam
automatically.

If you use multiple email accounts, then unlike many programs, Thunderbird
allows you to control junk mail differently for each account.

In Thunderbird, click on Tools, Account
Settings…, and then Junk Settings within the email account you want
to configure.

In the example above, I’m looking at the Junk Settings for my first
configured email account (if you have only one email account, it’ll be the first
as well ). “Enable” is checked to turn on the adaptive filter. I’ve also
specified that if the sender of an email is in my Personal Address Book, email
from that person should never be marked as junk.

Finally, I’ve indicated that junk mail should be automatically moved to a
Junk mail folder.

Training the Junk Mail Filter

Once you’ve got it enabled, you can start telling Thunderbird what is and is
not junk.

In the message list, there’s a small “flame” icon (enlarged here):

Looking in list view, you’ll see one of the column headers is that flame:

If the message has a flame icon and it’s not really spam, click on the icon
to unset it. If the message is clearly spam (as the examples above all are),
then if the icon is not set, click that message in that column to set it.
Thunderbird will now treat that message as spam and use it to ‘learn’ what you
think is spam.

Similarly, if you’re looking at a full message, there’s a Junk button
available to tell Thunderbird that what you’re looking at is spam:

Filtering specific senders

If there’s a specific email address from which you never want to see email,
just like Outlook Express, you can use message filters to dispose of the
messages as soon as they arrive. There is no way to prevent the messages from
being sent to you – again, regardless of what email program you use – but you
can make them disappear as soon as they show up.

On Thunderbird’s Tools menu, click on Message
Filters…:

Again, Thunderbird allows you to maintain a different set of filters for
every account you use, so start by selecting the account in “Filters for:”.
Then click on New…:

In the example shown above, I’ve defined a filter:

• It’s named “Delete mail from Fred.”

• It looks for “From” containing Fred’s email address
  (fred@somerandomservice.com).

• When it finds mail that matches that condition, it deletes the mail
  immediately.

With that rule, I’ll never see email from Fred again.

Naturally, you’d add whatever conditions and email addresses you would want
to block.

Click OK.

We now have a blocking filter.

Why is there no Block Sender…?

Block Sender… in Outlook Express is really just a shortcut that does the
equivalent of creating the filter we created above. (Outlook Express kept this
as a separate list – no actual filter was created – but it was essentially the
same thing.)

The problem is that Block Sender… doesn’t work.

More correctly, Block Sender… doesn’t do what most people think it
does.

Block Sender… doesn’t stop spam. It doesn’t even make a
dent in it.

Spammers caught wind of Block Sender… years ago, and they render
it completely ineffective simply by sending their messages from
hundreds of thousands of randomly chosen spoofed email addresses. The
“From:” line in spam is now completely meaningless, and it’s completely
pointless to try and block the supposed sender of spam.

Building a block senders list to stop spam only slows down your email
program as it has to check every email that comes in against the list.

And it doesn’t work.

It will not block, or even slow down, spam. The only thing it will slow down
is your email program.

Use the spam or junk email filters to deal with spam.

Create sender-blocking filters when you have an actual, specific email
address that you know is repeatedly sending you something you never
want to see.