Eu nunca vi essa opção, então fiquei curioso! Eu encontrei isto:
link
Burst Digests into Individual Messages - You can use this feature when you subscribe to daily message digests. This feature breaks the digest into separate and constituent messages. If you select this item, you can specify that the original digest message is deleted after it is burst.
An email digest is an email that is automatically generated by an electronic mailing list and which combines all exchanged emails during a time period (e.g. day, week, month, etc.) or when a volume limit is reached (e.g. every 10 or 100 messages) into one single message.
EDIT: Encontrou o seguinte também: link
When MH forwards message(s) with forw, or makes a digest with forw -digest, it uses a special format called bit stuffing. That format makes it easy for the recipient to extract the "encapsulated" messages, one by one. The MH burst command does this; it splits (bursts) the group into separate messages. Recent versions of burst also handle MIME-encoded digests (see the Section Forwarding in MIME Format, forw -mime).