SMTP incoming data timeout - closing connection

Hello,

My application is using Indy 10 components TIdMessage and TIdSMTP. I can
send an e-mail with a pdf attachment without any problem.
One of my clients can send an e-mail without an attachment without any
problem, but when he adds a pdf attachment, my application is waiting
for an hour and than comes with the following error message:
"mailrelay02.solcon.nl SMTP incoming data timeout - closing connection".

I use the following code:

     with TIdText.Create( IdMessage1.MessageParts, nil) do begin
       Body.Text := TwDHTMLEdit1.DocumentHTML;
       ContentType := 'text/html';
     end;
     with TIdAttachmentFile.Create(IdMessage1.MessageParts,
'Agreement.pdf') do begin
       ContentType := GetMIMETypeFromFile( 'Agreement.pdf');
     end;
     IdMessage1.ContentType := 'multipart/mixed';

How can this problem be solved ?

Thanks,

Hubert