You want to select mailboxes for archiving from an email client and you see a yellow triangle in front of an account. What does this mean?
The screenshot shows the yellow triangle in front of the account "Mothsoftware-Test".
Mail Archiver wants to archive ALL your email. The yellow triangle warns you that attachments may be missing if you archive an account with a yellow triangle.
If I go to Mail and check the settings for the "Mothsoftware-Test" account I see in Account Information that "Download Attachments" is set to "None".
After changing the value from "None" to "All" the yellow triangle in Mail Archive will go away. You will need to wait a bit before archiving until Mail has downloaded all attachments.
Outlook uses a bit older terminology. Here the setting is called "Download headers only". This needs to be OFF.
9-Dec-2023: updated with new screenshots.