I just migrated a Sharepoint subsite to its own site collection. The site has Sharepoint designer workflows that send emails in certain conditions. In the new site collection, none of my designer workflows will send emails. They always fail with the message

"The e-mail message cannot be sent. Make sure the e-mail has a valid recipient".

Any idea what might be causing this? I even created a new workflow directly for the new site collection with only one email action and it also fails with the same error.

Also, in the workflow email action dialog, when selecting the email recipient, the list box of available recipients shows a very small list of people and groups. What drives this list? In the old subsite, this list was very large, containing all users in my company.

Edit: Not sure if this is a clue or not, but in the old subsite, site permissions, there is an account listed here called "System Account (SHAREPOINT\system)" with limited access. This is the only difference I can find between the two sites. I tried to add this account to the new site, but sharepoint can't find that user.