1: 'Firewall -> Advanced -> Predefined Firewall Policies'.
2: Double click on 'Email Client' and click Add and there:
Action = Allow
Protocol = TCP
Direction = Out
Description = (as you want)
Source Address = Any
Destination Address = Any
Source Port = Any
Destination Port = 80
3: And click Apply and so on...
4: If you are lucky, this might work
You could also add port 443 in same way as above so TB could update add-ons and themes if you use such.
You have to move this rule(s) above the 'Ask' rule.