Need help solving SMTP mystery

I'm the developer for a site hosted at DiscountASP.

We have an email notification feature for subscribers to our website. It's nothing particularly fancy. It's an asp.net webpage where we enter data, click a button, and it uses the JMail component to email it out to about 275 addresses. One address per email - sometimes with one or more attachments such as pdf's or spreadsheets.

We've been using this solution for a few years now at DiscountASP. It's worked great for us until very recently.

In the last few days, we've noticed that the emails sometimes don't seem to be going out. Or at least we're not getting them ourselves. It's hit or miss. Sometimes we get them and sometimes we do not. Myself and the site owner are using different receiving systems/domains/isps and either we both get the mails or we both do not. I've not been able to track it down to a receiving problem such as spam rejection and I've not logged any errors with the send email script. I've seen no bounce message emails.

I've been tinkering with it since the weekend and tried changing the email component away from JMail to using System.Net.Mail and the behavior is the same.

One curious thing is that I can sometimes try to send them and don't get the email, but then change the subject text of the email and resend and then it does go out....but only once or twice and then i'll have to change the subject again. (We've been sending all of these, all these years, with the same email subject.) This happens with both JMail and System.Net.Mail.

Is there some sort of spam/bulkmail checking in place at DiscountASP that's sometimes putting a halt to the mails being sent? I understand that there is a 1000 per hour and 5000 per day restriction, but I don't think we're hitting those levels.

Beyond solving this immediate problem, I'd welcome any advice on what's considered best practices/components for doing emailing of this sort at DiscountASP.

Many Thanks,
-CL