We have an exchange server, and are having problems when sending email to three of our customers.
When there are no attachments the email is delivered successfully, when a pdf (from a word doc) is attached, size around 1mb, the email is delivered successfully, but when the attachment is a pdf from an autocad drawing, size only 500kb the email fails and nobody gets an NDR.
The problem started around 2 weeks ago, all fine before that. I was recently asked by our ISP to turn off NDRs due to backscatter. We are sending through smtp.blueyomder.co.uk, I wondered about antispam filter/blacklist issues? I can successfully send an email with a 9.5mb zip file containing pdfs to a gmail account and a yahoo account.