SEDNA limits the size of email that can be sent or received as follows:
Inbound Email - 30 MB, including attachments
Outbound Email - 20 MB, including attachments
Note: Message size, including file attachments, is limited to 18MB when composing to allow for any compression rules that may inflate the message above 20MB. An error will appear if the message exceeds 18MB.
SEDNA provides a feature to allow larger volumes of data to be shared externally, including the ability to share large numbers of emails. Details on this feature can be found in this knowledge article.