Hi Carlos.
Thanks for the reply. I think the problem though is at this end rather than the users. I tested the bulk operations you suggested and again I received the email in exchange, and hotmail with no attachment.
I turned verbose debugging on though and I have errors. Here are the issues in the log I can see which might be causing the problem - What do you think? ....I've commented at the end of the debug message btw.....
Debug:
Could not access file: E:\web\gravyforthe\htdocs\moodle\App_Data/E:\web\gravyforthe\htdocs\moodle\App_Data\temp\mod_simplecertificate\Tes8A1F.tmp
SMTP -> get_lines(): $data was ""SMTP -> get_lines(): $str is "220 sm10.internetmailserver.net
"SMTP -> get_lines(): $data is "220 sm10.internetmailserver.net
"SMTP -> FROM SERVER:220 sm10.internetmailserver.net
CLIENT -> SMTP: EHLO www.gravyforthebrain.com
-----blah blah blah------
"SMTP -> get_lines(): $data is "250 OK
"SMTP -> FROM SERVER:250 OK
CLIENT -> SMTP: QUIT
SMTP -> get_lines(): $data was ""SMTP -> get_lines(): $str is "221 Service closing transmission channel
"SMTP -> get_lines(): $data is "221 Service closing transmission channel
"SMTP -> FROM SERVER:221 Service closing transmission channel
TCPDF ERROR: Some data has already been output, can't send PDF file
-----
My guess is that this is a bug as it is trying to find:
E:\web\gravyforthe\htdocs\moodle\App_Data/E:\web\gravyforthe\htdocs\moodle\App_Data\temp\mod_simplecertificate\Tes8A1F.tmp
....when it should be trying to find:
E:\web\gravyforthe\htdocs\moodle\App_Data\temp\mod_simplecertificate\Tes8A1F.tmp
I'd be grateful for your help on trying to fix this... 
bulk certificate creation, for all students (or other roles), some problems:
but why not let the students issue their own certificates? you want to print it and send to your students ? , if no i don't see any benefit to do that.