1. Configure a user with an alternate email address

2. Configure in user preferences receive forum posts as a digest

3. Subscribe the user to a forum and post a message

4. The user will receive the digest in the email address configured in his/her profile, not in the alternate address

In case that user doesn't want a digest, the process ends calling function "send_message", in "message/output/email/message_output_email.php", which configures the email to be the alternate address.

If the user wants a digest, in "message/output/lib.php", line 731, the post is stored in the "forum_queue" table and stops processing it. This post, with the other in the same digest, will be sent later in line 1183 by the function "email_to_user" (defined in "lib/moodlelib.php"), which sends the message directly to the email in user's profile.

I think that the easiest solution could be that function "email_to_user" check if user has an alternate address to receive the emails.