There are two possible warnings that may appear on an email as part of Bell Mail security.
Yellow warning: Indicates that the message is not from someone on your contact list. Links and pictures in the message are blocked, and can be unblocked by clicking Show content.
If the message is from someone you trust, you can click Mark as safe in the message header to add the sender to your safe sender list.
Red warning: Indicates a message that has been blocked for your security. Delete any suspicious-looking email messages without opening them.