Data type supported in custom workflow activities in CRM

The following types are only supported in input and output argument.

• Bool 
• DateTime 
• Decimal 
• Double 
• EntityReference 
• Int 
• Money 
• OptionSetValue 
• String 

Note:

Collection or Array of object as output parameter is not possible. But we can do entityreference list but that also not work when we map the multiple recipient in email.

https://msdn.microsoft.com/en-us/library/gg327984.aspx