MAPI - 50% of receivers don't receive attachment

[robinvb]

Hi guys!

We have a program (developed in Clarion, sold to clients), that has an email functionality (submit Request for Quotes, etc.). By default, our clients can set up their email settings as SMTP, but it also supports MAPI as not every mailserver allows SMTP. MAPI is behaving weird though:

The email is plain text, and contains one PDF attachment (about 200 kb). There is one email address in the "TO"-field (us), and all the recipients are in the "BCC"-field (suppliers).

Some suppliers (about 50%) do not receive the attachment, and there is no notification in the body of the email. If then I go to Outlook, Sent Items folder, and reopen the email, all recipients are there. If I say "resend", they all receive it fine.

All that our program does is "call" the MAPI component. What-o-what could this be?

Thanks!