FIFTH AMENDMENT TO LEASE AGREEMENT

QuickLinks -- Click here to rapidly navigate through this document

Exhibit 10.21

FIFTH AMENDMENT TO LEASE AGREEMENT

        THIS FIFTH AMENDMENT TO LEASE AGREEMENT (the " Amendment ") made this 20 ^th  day of August, 2006 between DAN ROAD BUILDING LLC, a Delaware limited liability company, with an address in care of Great Point Investors LLC, Two Center Plaza, Suite 410, Boston, MA 02108 (" Landlord ") and NETWORK ENGINES, INC., a Delaware corporation, with an address of 15 Dan Road, Canton, MA 02021 (" Tenant ").

        WHEREAS, New Boston Batterymarch Limited Partnership, Landlord's predecessor-in-interest, and Tenant entered into a Lease Agreement dated October 19, 1999, as amended by a First Amendment to Lease Agreement dated February 1, 2000, a Second Amendment to Lease Agreement dated June 1, 2000, a Third Amendment to Lease Agreement dated September 14, 2000 and a Fourth Amendment to Lease Agreement dated October 20, 2003 (as amended, the " Lease "), pursuant to which Tenant leases approximately 51,935 rentable square feet at 15 Dan Road, Canton, Massachusetts (the " Premises ");

        WHEREAS, Tenant desires to extend the initial term of the Lease for the Premises through March 31, 2009, and Landlord has agreed to such extension on the terms and conditions set forth herein; and

        WHEREAS, Landlord and Tenant desire to memorialize their understanding and modify the Lease consistent therewith.

        NOW, THEREFORE, in consideration of the mutual covenants herein contained, and other good and valuable consideration, the receipt and sufficiency of which are hereby acknowledged, Landlord and Tenant agree that the Lease shall be amended as follows:

        1.      Term .    The Lease Term shall be extended so as to include the period from March 31, 2007 through March 31, 2009.

        2.      Rent .    Annual base Rent for such extended period shall be as follows: April 1, 2007 through March 31, 2008: $662,171.25 per annum ($55,180.94 per month); and April 1, 2008 through March 31, 2009: $714,106.25 per annum ($59,508.85 per month). Notwithstanding the foregoing, Tenant shall not be required to pay base Rent for the month of April, 2007.

        3.      Security Deposit .    As a condition to the effectiveness of this Amendment, on or before October 31, 2006, Tenant shall deliver to Landlord an amendment to Letter of Credit No. SVB00IS1965 issued by Silicon Valley Bank, modifying the expiration date of such Letter of Credit to April 30, 2009.

        4.      Renewal Option .    Section XLX of the Lease is hereby replaced with the following with respect to Tenant's option to extend the Term of the Lease beyond March 31, 2009:

        "Provided that Tenant is not in default beyond any applicable grace or cure period hereunder, Tenant shall have the right to extend the term of this Lease for one (1) additional term of two (2) years (the " Extended Term ") commencing on April 1, 2009. Tenant shall exercise the right to extend the term of this Lease by written notice to Landlord no later than June 30, 2008. The Rent for the Extended Term shall be the greater of the then current Rent or the Market Rent as hereinafter defined. The " Market Rent " shall be determined by Landlord based on leases then currently under negotiation or recently executed for the Property. If no such leases are under negotiation or have been recently executed, then the Market Rent shall be reasonably determined by Landlord based on comparable office space in the town or city in which the Premises are located (if any such space exists) taking into account, among other factors, amenities, setting, location and de