Email cannot be sent from an alias. Aliases are not real email accounts. They can only forward email to another email address.
Lets say, for example, that your alias account, president@yourassociation.com, forwards emails to your personal accout, myname@hotmail.com. This means that when somebody emails president@yourassociation.com, that email will appear in the inbox at myname@hotmail.com.
To reply to emails sent to email aliases, find them in the inbox of the address the alias forwards emails to.