504 suspicious message!!!

[#1]

OOoo errrr!
Ok, so I've recently got back into a bit of programming and decided to pick up on my smtp server for handling incoming emails. Now as I'm sure you'll probably notice from your logs I've been sending quite a lot of test emails to the same address (the domain is a ddns from no-ip.com).
Anyway I've until now been using a large file in outlook express with the setting to break the file apart in order to send myself large numbers of emails (so I can test out the multiple threads, form updates etc). Anyway I've today decided to observe the memory usage via the task manager (yes I know there are better ways but this is basic stuff) and decided to send just one email with a 3.5Mb file so that I can see how much memory is used and whether its 3.5Mb or just the connection objects created when the connection is initiated.
I hit a snag.. Outlook express delivered me a 504 suspicious message error! Now I used to use this account to send lots of photos to a website editor about railway related stuff so I know you guys can handle large emails but I was just wondering why an email thats smaller but with a pretty large file size still gets rejected?

[#2]

Nevermind, I tried again with the file in an archive (it was an exe - first convenient file I could find of that size) and it sent perfectly well.
You can ignore this thread if you like 

[#3]

Outlook Express tends to regard .exe attachments as suspicious when they are received, perhaps it does the same when sending. Are you sure the 504 error you had was not generated by Outlook Express itself with the message not even getting to the Plusnet Cloudmarks? Zipped archives containing .exe files are normally accepted.