MessageData represents eml (MIME) data for specific email message downloaded by IMAP client.
MailBuilder is used to create a new IMail instance, either by parsing eml data -or- by creating new email from scratch.
To obtain IMail having MessageData use:
MessageData messageData = ...;
IMail email = new MailBuilder().CreateFromEml(messageData.EmlData);
If for some reason you want to create MailBuilder from existing IMail use IMail.ToBuilder() - it creates new MailBuilder with all data imported from the original email object. It can be used to create a new email.