The mail specification offers no guarantee of delivery. It never has. But looking to the icons in Thunderbird will not assist you with delivery failures.

Where you get no notification of delivery failure it is almost always because a spam filter on the receiving end has binned your mail. Given the totally arbitrary spam rules some places have in place it becomes a field all unto itself trying to understand why mail did not get there. Common issues involve images especially as the last thing in the email, lost of images with links (think social media polluted signatures) or things a minor as sending the mail without a subject. Common for the SMS addicted younger folk, but almost a guarantee of delivery problems to some servers. Just as BCC mail is an issue with some providers. (Lots of SPAM is BCC'ed )

"Speculative user comments." Sounds like the comment the last person that asked this question used. So similar in fact I checked to see if you had asked the question before on this account. Left pointing arrows are "forwards" and right pointing are "replies". The folder is not in any way important the same arrows are used in message lists throughout.

The meanings have remained constant version 2 even is the appearance has has some tweaks.

It is also a fact that depending on the "theme" you use, the appearance of those arrows is also included in the theme. Each operating system has it's own theme, so the arrows do not look the same on windows, Mac and Linux. Similar, but not the same exactly. That is before users are allowed to change the theme they use in the add-on manager. So a definitive "this arrow means this" is about impossible to do as the exact detail changes depending on the operating system and theme used.

Thank you. My Sent box shows few arrows in any direction, notwithstanding that many messages are replies, some are forwards, and a few are kinda both. This drove me to speculate about possible other arrow meanings such as (inherently fragmentary) delivery status.

I do understand that the wonderful world of Internet E-mail often leaves a message's delivery status a mystery to me. That's the way of it. (In contrast, experience teaches that proprietary, closed systems (think Notes, GroupWise) handle this very well.)

But I don't understand why Thunderbird shows no arrows for many of the replies in my Sent box. fwiw, I'm running version 52.9.1 on CentOS 7 with KDE 4.1 and the CentOS7 theme. However, it's of little consequence.

I appreciate your responding even though this question has been previously addressed. I don't think I've asked *any* question before. It's possible my verbiage was influenced by reading what others have written.

Best regards, and thanks again.

It is also worth noting that some IMAPp mail servers do not support the forward and reply flags, so they disappear when he folder is synchronized. That may be why you only see "some"