@PhilS100 Thanks for your post. The error code 550 5.7.1 usually means the sending mail server has been blocked by our platform due to a security or reputation issue on their side — it’s not related to your Safe Senders list.
There’s nothing you need to change on your account, but your sister’s email provider (Plusnet) will need to check their outgoing mail server and request delisting if it’s been flagged. They can do this by contacting their own support team to investigate the rejection and resolve the server reputation issue.
