URL Logo in Signature Not Updating

Our company recently updated our logo in our signatures which is connected to a URL. Some of our devices are receiving the updated logo, while some are not. It seems that devices that are being newly setup with company email on the mail app are receiving the new logo, while devices that have already been configured are not.

Is there an option to delete cache on the mail app or the iPhone which would allow the updated URL logo to show up? I've tried deleting the email account and re-adding which does not work. Restarting the phone does nothing and resetting the phone to an out of box state is not an option.

The device model does not matter for this issue.