8MB of attachments is not a big message - Mail.dll should handle that just fine.
How does your code look like?
Can you create a sample project (possibly with test attachments) that reproduces this issue? Please zip it and send it directly to our support (you can find the address in the page footer).
I run a simple test just now:
MailBuilder builder = new MailBuilder();
builder.AddAttachment(new byte[3000000]); // 3 MB
builder.AddAttachment(new byte[3000000]); // 3 MB
builder.AddAttachment(new byte[3000000]); // 3 MB
IMail mail = builder.Create();
byte[] eml = mail.Render();
File.WriteAllBytes("test.eml", eml);
It results in ~12MB email. Memory usage is nowhere near the amount that would cause OutOfMemoryException.