Client receives suspicious link warning when I send a payment collection email

[enduringfruits]

Hi, when I send out an invoice or even a welcome email, the client receives the email with red highlights and text that says "suspicious link" or something similar.

I'm not sure if this is just a one off thing because of this client's computer, but I want to make certain. The warning is from norton but the client said he uninstalled norton.

Does anyone know why this is happening and how I would go to prevent this?

Thank you.