Surabhi: upon re-reading your post, it is not clear if you have ever sent an email from the mainframe using SMTP before or not. If not, you need to start with a simple text message -- something like
If this goes to your email client, then you know you've got the connection correct. If you copy this to (B,SMTP) and you don't get the email reasonably quickly (I'd allow 30 minutes or so -- it's usually seconds but that will depend on the site and the SMTP traffic of the site), then you'll need to work with your site support group as something basic isn't right. That could include line numbers in 73 - 80 (SMTP does not understand line numbers), wrong SYSOUT class, wrong started task name, mainframe pointing to wrong mail server, mainframe not pointing to a mail server at all, or various other set up issues. If you don't understand the format listed here, you'll need to find RFC2821 and RFC2822 about SMTP to learn more -- the HELO, MAIL FROM:, MAIL TO: and DATA lines have a particular format to follow. And, as I said earlier, the final period is not optional for SMTP -- some sites automatically put it in for you but many do not.
Once you're getting emails from the mainframe, then you can focus on attachments.