For those of you with WIndows Mobile 6.1 devices, you might notice that your Activesync emails all automatically have signatures. We've gotten requests from partners wanting to do add/modify the signature to an account as well.  Here is a sample function to update the signature for the Activesync account.  Error checking removed in this sample code :).

#include <atlbase.h>
#include <cemapi.h>

HRESULT AddSignatureToAccount()
    HRESULT hr;

    CComPtr<IMAPITable> ptbl;
    CComPtr<IMAPISession> pSession;
    CComPtr<IMsgStore> pMsgStore;

    // Log onto MAPI
    hr = MAPILogonEx(0, NULL, NULL, 0, static_cast<LPMAPISESSION *>(&pSession));

    // You can open a different message store here instead of the default
    hr = pSession->OpenMsgStore(NULL, 0, NULL, NULL, 0, &pMsgStore);

    SPropValue rgspv[3] = { 0 };

    rgspv[0].ulPropTag      =   PR_CE_SIGNATURE;    // signature content
    rgspv[0].Value.lpszW    =   L"Sent from my personal Windows Mobile phone";
    rgspv[1].ulPropTag      =   PR_CE_USE_SIGNATURE;    // use the signature in newly composed emails
    rgspv[1].Value.b        =   TRUE;
    rgspv[2].ulPropTag      =   PR_CE_USE_SIGNATURE_REPLY_FORWARD;    // use signature in replied or forwarded emails
    rgspv[2].Value.b        =   TRUE;

    // save the data the properties
    hr = pMsgStore->SetProps (3, rgspv, NULL);
    // Log off
    pSession->Logoff(0, 0, 0);

    return hr;

This should be a good starting point on how to set properties on message accounts as well. You can find a list of valid properties in mapitags.h.  Have fun!