I'm having trouble delegating administrative rights to manage send-as permissions on public folders. I've added the account to the public folder administrators group and the recipient administrators group but they still get access denied. I wonder if the permissions giving to these groups changed? What rights are needed to be able to manage send-as permissions?

HelloCorrect me if Iam wrong. I believe you are trying to configure public folders so that user can send mail on the public folder's behalf. Rit ?If yes, please refer "Designating a User as a Public Folder Delegate" in :http://technet.microsoft.com/en-us/library/aa996122(EXCHG.65).aspx

No, I'm trying to give administrator rights to manage the send-as permissions over public folders. Right now, when they try to grant send-as rights, they get access denied.

Permissions which you gave looks good. Did ou restart the Information store to make it replicated ?If not try restarting the Information Store after adding the account to the "Public folder administrators group" and "Recipient administrators group"and then try giving Send-as.