One of the best reasons to use the runtime TOpions of EmailSettings is User Secrets. There is plenty of good documentation on the User Secrets configuration and setup. See MSDocs - Safe storage of app secrets in development in ASP.NET Core. This article will demonstrate applying, naming, and sharing User Secrets.

Use the Secret Manager tool to create a configuration folder named with a random GUID and an empty secrets. json file by right clicking on the project name then click Manage User Secrets.

Add an EmailSettings section to secrets. json with your email server settings. If you want to use Gmail for SMTP, you must allow less secure app access to the Google account.

"EmailSettings": {
  "Configured": false,
  "Timeout": 30000, // default = 120000 = 2 minutes
  "MailServer": "smtp.gmail.com",
  "MailPort": 465,
  "SenderName": "Sender Name",
  "SenderEmail": "senderemail@gmail.com",
  "Password": "GmailPassword",
  "PasswordEncrypted": false,
  "BannerBackcolor": "#FF0000",
  "BannerColor": "#FFFFFF",
  "BannerText": "User Secrets",
  "EmailSignature": "Email Signature",
  "SupportName": "Secret Support",
  "SupportEmail": "supportemail@gmail.com",
  "AdminEmail": "adminemail@gmail.com"
}

The Secret Manager tool hides implementation details, such as where and how the values are stored. You can use the tool without knowing these implementation details. The values are stored in a JSON file in the local machine's user profile folder:

%APPDATA%\Microsoft\UserSecrets\<user_secrets_id>\secrets.json

Where <user_secrets_id> is the UserSecretsId value specified in the project file.

I use the same secrets. json configuration for the development of multiple projects. I manage different User Secrets configurations by naming them with a friendly name and last update date like ken-gmail-20210801. I locate the secrets. json file and rename the containing folder. Then I update the UserSecretsId value specified in the project file.

<UserSecretsId>ken-gmail-20210801</UserSecretsId>

I share the named User Secrets across multiple projects by adding or updating the UserSecretsId.