They mean forwarded. Obviously you did not read the link I gave you on the last post you made or you would have got that.

Yes I did read the link! You are making unjust accusations again! If you read my question properly and look at the screen shots you will see that there are no blue arrows at any forwarded messages and the ones with blue arrows were not forwarded but had issues about recipients not receiving them!

Regardless of the meaning you would like to attribute a blue arrow means forwarded by you to someone else. All the messages with FWD in the subject are messages someone has forwarded to you.

This information is still as relevant as when I posted it before.

Based on your past question I am guessing you are not getting replies to email and either are assuming they were not delivered or that they were not sent. I suggest you approach your mail provider. Those that operate the server. They have logs of what mail they delivered to what servers, or what delivery attempts failed. My guess is you may either have been blacklisted as a spammer or your emails look like spam to the receiving server. But it is always possible that your provider simply made no attempt to deliver your mail once it got to their SMTP server, or your recipients have simply refused delivery of further mail from your address. None of those things can be cured anywhere but in consultation with your mail provider.

Am I repeating myself. Yes I am.

The style and colour of such symbols will vary with your platform, theme and possibly version of Thunderbird.

Here, using TB 52.9.1 with the Phoenity Shredder theme on Windows 10, I get blue leftward-pointing arrows to signify a reply, and purplish rightward-pointing arrows to indicate a forwarded message.

In safe mode, the same version of Thunderbird, again on Windows 10, using (presumably) the default icons, I get a leftward pointing purple arrow on replies, and a blue right-pointing arrow for forwards.

Either way round, an arrow pointing to the right seems to mean "forwarded".

At the point of sending, Thunderbird won't know if a message was successfully relayed to the intended recipient. It's unlikely that the icon shown here bears any relation to the success or otherwise of the message's delivery. You'd normally get a separate delivery failure notice if your message is refused. The usual reasons are an invalid or expired email address, a closed account, a full inbox or a (usually temporary) outage of the receiving server.