IRM (IRS Manual) 1.15.6 Managing Electronic Records Found here
-------------
1.15.6.6
Standards for Managing Electronic Mail Records ...
3. IRS offices will not store the official recordkeeping copy of e-mail messages that are federal records ONLY on the electronic mail system, unless the system has all of the features of an electronic recordkeeping system, some of which are specified in paragraph 2 above. If the electronic mail system is not designed to be a recordkeeping system, ask an E-Mail/System Administrator to instruct you on how to copy the information from the electronic mail system to a recordkeeping system or produce a hard copy[emphasis mine] for recordkeeping purposes.
4. IRS offices that maintain their e-mail records electronically will move or copy them to a separate electronic recordkeeping system unless their system has the features specified in IRM 1.15.6.6.2 above. Backup tapes are not to be used for recordkeeping purposes.[emphasis mine] ...
6. Offices that maintain paper files as their recordkeeping systems will print their e-mail records[emphasis mine] and the related transmission and receipt data. ...
Exhibit 1.15.6-1
Common Questions about E-Mail
Are there special requirements for retaining e-mail messages as records
The basic requirements applicable to all records apply to e-mail records as well. If they are not in an approved electronic recordkeeping system, then the e-mail messages identified as records must be printed out and placed in the appropriate record system[emphasis mine]. However, there are some specific elements for records sent or received through e-mail which also must be captured in addition to the message to satisfy recordkeeping requirements. You should ensure that...
--------------------
There are a bunch of other IRS manuals that discuss printing, filing and retaining records. The issue here is that there is a requirement, formalized by statute and regulation, that all government agencies retain records.
Now about your guarantee; it doesn't say that each and every email has to be printed. It says that if there isn't a suitable electronic recordkeeping system then it has to be printed.