A POP3 server uses Amazon S3 bucket as a mailbox and implements RFC 1939.
dotnet user-secrets set AWS:AccessKey ACCESS_KEY
dotnet user-secrets set AWS:SecretKey SECRET_ACCESS_KEY
dotnet user-secrets set AWS:Region REGION
export AWS__ACCESSKEY=ACCESS_KEY
export AWS__SECRETKEY=SECRET_ACCESS_KEY
export AWS__REGION=REGION
Keep secure the server by running it behind a firewall and whitelisting client ip addresses.