Default Exchange 2007 limit for TO/BCC/CC is 5000

In order to check you r configuration run on Exchange Management Shell Get-TransportConfig | fl
You will receive the following output

ClearCategories                : True
ConvertDisclaimerWrapperToEml  : False
DSNConversionMode              : UseExchangeDSNs
GenerateCopyOfDSNFor           : {5.4.8, 5.4.6, 5.4.4, 5.2.4, 5.2.0, 5.1.4}
InternalSMTPServers            : {}
JournalingReportNdrTo          : <>
MaxDumpsterSizePerStorageGroup : 18MB
MaxDumpsterTime                : 7.00:00:00
MaxReceiveSize                 : 30MB
MaxRecipientEnvelopeLimit      : 5000
MaxSendSize                    : 30MB
TLSReceiveDomainSecureList     : {}
TLSSendDomainSecureList        : {}
VerifySecureSubmitEnabled      : False
VoicemailJournalingEnabled     : True
HeaderPromotionModeSetting     : NoCreate
WritingBrandingInDSNEnabled    : True
Xexch50Enabled                 : True

Check out MaxRecipientEnvelopeLimit line to see your exchange setting

In order to reduce or increase the limit type the following command

Set-TransportConfig –MaxRecipientEnvelopeLimit 10000

Where 10000 set required number of connections