To increase mail attachment sending limits look in one of two places.

1. Active Directory – domainName.nnn – Users – userName properties – Exchange General tab – Delivery Restrictions Button. Under the Sending or Receiving message size areas click the radio button for “Maximum KB” and set as desired. 1000 KB is approximately 1 MB (20,000KB = 20MB)
2. Exchange Server Manager: Under the “First Organization (Exchange)” open the Global Settings folder, then Message Delivery, and then Defaults to set the limit.