Spanning Backup can be used as an archival solution for email, as we will never delete any emails unless you ask us to set up an email retention policy for you. In the absence of an email retention policy, Spanning Backup will never (and you can never) delete anything that's been backed up as long as the account is paid and active. Click here for more info about the email retention policy.
That being said, Spanning Backup does not explicitly meet FINRA/SEC 17a-4 requirements for email archiving in the financial services industry. Those regulations require real time archiving of client communications and the storage of those communications on permanent (WORM) storage devices, and Spanning Backup meets neither of these requirements.
We should note that G Suite and Google Vault also don't meet the FINRA requirements, and Spanning actually plugs many of their holes regarding this. It is also possible that in a cloud computing environment, that Spanning Backup may pass a FINRA review. However, we have not explored this beyond discussions with some of our financial services customers.