I'm not aware of any Sage 100 specific solutions, but there are a ton of IT-based options to do this. If they have decent in-house IT, they could set up an SMTP Relay server that establishes a TLS encrypted connection between them and the bank. Or if they happen to use a cloud-based spam-filtering service (GFI, Postini, McAfee, etc), TLS might be one of the out-of-the-box options (usually configured by their email domain). For more commercial options, there are things like LeapFile, RPost, ZixMail, or Office 365 Azure Rights Management.