It's most likely a bug in the product. Unfortunately, Microsoft discontinued support for Microsoft OutLook Express many years ago and there have been no updates since, so if your Windows Update check doesn't bring in any remaining updates for OutLook Express then the chances of this bug ever getting fixed are likely zero. =(
Good news: There's a free and open-source alternative to OutLook Express...
For folks who like the MS-OutLook Express interface, switching to Mozilla Thunderbird is generally a very helpful progression because it defaults to the same layout of folders and whatnot, but also with some additional security features. During installation, Thunderbird will also provide an option to import from OutLook Express.
Many folks even regard Thunderbird as a natural upgrade step for OutLook Express users. It's important to note that OutLook Express has some serious bugs that could result in data loss (e.g., mailboxes with 16,383 messages act like they store additional messages but in reality lose them immediately), so switching to an alternative is strongly recommended.
Mozilla Thunderbird (free and open source)