User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/87.0.4280.141 Safari/537.36

Steps to reproduce:

Normal use of Thunderbird 78.7.0 (32-bit) on Windows 10, reading mail in POP3 account in Classic View

Actual results:

A new message is shown in the message list with no icon in the Attachment column. After I click on the message to read it, the attachment icon appears and it becomes obvious there's an attachment.

This is intermittent behavior. It happens only occasionally and I'm sorry I don't know how to reproduce. But I have observed it for months over different versions of Thunderbird on Windows. (For what it's worth, the most recent example came from Apple Mail, with two attachments, and when I forwarded that message it showed four attachments, two of which were html documents representing the message content.)

Expected results:

When the message is received, Thunderbird should immediately recognize there is an attachment, and the attachment icon should be displayed in the message list.

Hello Brett, thank you for your bug report. This is part of an old story. It relates to the MIME structure of the message, and there's a lot of different Mime structures out there... Technically, we need to read the message and parse it before being able to truly say if attachments exist... I guess that for performance reasons, maybe we don't always pre-parse all messages - although not sure why we couldn't use empty cycles to that in the background (and maybe we do, saw some comments to that end involving GLODA indexing). So there's also a time factor, that the attachment icon might be wrong/missing initially but may fix itself later. Anyway, in terms of bug management, I'll dupe this to another general case.

I've filed META bug 1691191 as a central access point for all similar bugs, so maybe that'll help to figure things out...