It appears that Sylpheed, the default mail software for Lubuntu 18.04, deletes mail rather than archiving it, but I am still trying to verify that I'm not overlooking some hidden option.
What software should I use to archive mail rather than delete it? I also wish to view multiple accounts' mail together, rather than clicking mailbox by mailbox. Basically, I like macOS's Mail application, which enables users to view mail separately or collectively, and to archive or delete.
Mozilla Thunderbird is one package that will let you archive mail (See: https://support.mozilla.org/en-US/kb/archived-messages).
You can also combine messages from different POP accounts in a global inbox (See: https://support.mozilla.org/en-US/kb/unify-your-pop-email-accounts-global-inbox).