Use the ecoDMS Thunderbird plugin to archive incoming and outgoing emails from Thunderbird. ecoDMS archives the complete email, including any attachments, in the standard mail format EML. The message content (without attachments) is additionally filed as PDF/A.
In Thunderbird there are various functions available:
Details about macOS: No PDF/A files are generated via the ecoDMS Thunderbird plugin for macOS. Only the original e-mail is deposited as an EML file. This is not an error in ecoDMS: https://bugzilla.mozilla.org/show_bug.cgi?id=675709