The problem is that your smtp settings previously entered in Site Settings were encrypted with your previous machine key and now cannot be decrypted with the new one so it throws an error. This problem should affect all sites that have smtp settings entered in site settings.
There are 2 ways the problem can be solved.
1. Temporarily restore the previous machine key, then go into site settings copy your smtp user and password and then clear them out, then restore the new machine key and re-enter the smtp user and password in site settings. If you don't have a record of the smtp settings for each site anywhere else then this is the way to go.
2. You could clear out those settings with a database query then restore them in Site Settings so they will be encrypted with the new machine key. This solution is only possible if you know what those settings are for each site.
SET KeyValue = ''
WHERE KeyName = 'SMTPUser' OR KeyName = 'SMTPPassword'
Hope that helps,