A SWAG, but do you have reverse DNS set up at your site? It's not that widely used (yet) but some mail servers reject mail if you don't have it set up (or at least it counts as a demerit, and if a message has too many demerits it'll get rejected).

Are you able to send regular/conventional e-mails to those "problem" addresses? If so, how does the sending process differ between regular mails and your app?

>Rick,
>
>Thanks for looking at this.
>
>The message is: 451 451 4.7.0 Timeout waiting for client input (state 18).
>
>I doubt it's an AV issue. It always fails on the same two customers every time. All emails with Excel attachments successfully go out to the other 98 customers. We think it's a firewall issue or something at the customer site.
>
>I hate to say this, but rather than rewrite/fix in VFP I'd probably rewrite in .NET. The program is not that complicated. It's just an automation app that looks in a table for "Ready" rows of data and emails to the recipients in the table an attachment (Excel) that is on a fileserver. But, even in .NET I'll bet we get the same error.
>
>I think the customers email server doesn't like the fact that we are relaying from SMTP to our Exchange server or something like that.
>
>>It's hard to say what might be the problem. Most likely some proxy or AV issue...
>>
>>I'd still recommend looking at a more recent version and using the .NET module as it handles many more mail server scenarios than the old DLL based version does. You might get additional error info that might point at the problem.
>>
>>What is the error message on those failures anyway?
>>
>>+++ Rick ---
>>
>>>Rick,
>>>
>>>We are sending first to a SMTP server from VFP9 using a pretty old version of wwipstuff and the it goes out the door via our Exchange server. So, as a protocol SMTP is correct, I believe.
>>>
>>>Again, the failures are only to 2 customers out of 100. All 98 other emails to our customers successfully go through.
>>>
>>>>How are you sending the emails exactly? All going through the same mail server or sending to different mail servers?
>>>>
>>>>wwIPStuff only worked with a few protocols. wwSmtp (which is the later version with the same interface) supports a much wider variety of protocols including support for SSL connections for servers like gmail, outlook.com etc.
>>>>
>>>>The error looks like the server send some command that the client doesn't understand and subsequently hangs. If you use wwSmtp especially in .NET mode this is very likely to go away.
>>>>
>>>>http://west-wind.com/webconnection/docs/_s9001zxj2.htm
>>>>
>>>>+++ Rick ---