Unable to send an excel macro attachment

Unable to send Excel file with macro to a certain email addresses using pop3 server(example johndoe@example.com or JaneDoe@example.com).  I can send a regular excel attachment with no macro and it sends fine to johndoe@example.com or janedoe@example.com (it takes about an hour to get there).  I can send a test message with no attachments and it gets there in less than one minute.  Yet, when I send to other email addresses (johndoe@domain2.com or janedoe@domain2.com or john@domain3.com), the emails with macro attachments get sent ok (1 minute).  When I call the Pop3 support (windstream.net)  they say that it is a problem with our servers (we are using exchange server).  Even the senior support for windstream.net says that it is our server.  I ask our Senior administrator and she says its their server.  What server can it be?  We are able to receive excel macros from other domains just fine.