Support multiple to email addresses

Is recipient a string?
```
>>> list("esoergel@dimagi.com")
['e', 's', 'o', 'e', 'r', 'g', 'e', 'l', '@', 'd', 'i', 'm', 'a', 'g', 'i', '.',
 'c', 'o', 'm']
```
Might need something like
```python
if hasattr(recipient, "iter"):
    recipient = list(recipient)
```