Mail senders from one specific host will not go through to our Exchange 2010

Customers of one big email hosting company do not get through to our server. The hosting company have many servers, but the common denominator of all the people having trouble sending us mail, are that they are sending through one of their servers. As we are not customers of this hostingcompany, I don't get through to their technical staff, so I have to figure this out from my end.
What the SMTP-log tells me, is that the sending server sends an EHLO, a Mail From:, and then QUIT.  Mail sent from any other servers work perfectly to my knowledge.

My system is an isa 2004 server in front hosting a microsoft SMTP service on a 2003 server, forwarding to a Norman Email Protection Server as a smart host, in turn coupled to an exchange 2010 server.

Excerpt from the SMTP-log:

2014-05-08 00:10:16 212.18.128.233 SendersSmtpserver.com SMTPSVC1 MyIsaServer 12.12.12.12 0 EHLO - +SendersSmtpserver.com 250 0 242 19 0 SMTP - - - -
2014-05-08 00:10:16 212.18.128.233 SendersSmtpserver.com SMTPSVC1 MyIsaServer 12.12.12.12 0 MAIL - +FROM:<sender@sendersdomain.com> 250 0 78 45 16 SMTP - - - -
2014-05-08 00:10:16 212.18.128.233 SendersSmtpserver.com SMTPSVC1 MyIsaServer 12.12.12.12 0 QUIT - SendersSmtpserver.com 240 32 78 45 16 SMTP - - - -