When I try to add Sender Email in SendGrid credentials, there's no dropdown list with my email address. I can't save changes if I type the email address manually. Why?

Try to do the following: open your SendGrid account and check if your email address is verified (Settings > Sender Authentication). Then open your API Key settings (API Keys > Actions > Edit) and check the restrictions applied to the API key. The "Sender Authentication" row needs to have Full Access for the dropdown list to appear in the SendGrid credentials.