Email Distribution Groups and Send Mail as

Email Distribution Groups and Send Mail as

Good Morning,

I am writing because we have created a email distribution group and I have enabled the members of this group to be able to send mail out as that mail alias, however there isn't a drop down in the "from" field for sending mail as an authorized alias.  Please advise as to what is causing this issue, and if there is a different way it requires to be setup.

Thank you!