Sent email blocked by ISP and now trapped in an error loop

One of my emails apparently is being blocked as spam by RCN. I keep getting this pop-up error box:

An unknown error has occurred.

Subject 'Photos from trip 2'
Server Error: 554
Server Response: 554 5.7.1 [P4] Message blocked due to spam content in the message.
Server: 'smtp.rcn.com'
Windows Live Mail Error ID: 0x800CCC6F
Protocol: SMTP
Port: 587
Secure(SSL): No

Unfortunately I can't move past this error and have no way of deleting this email so the unsent emails behind it can be sent. I have dealt with RCN for over an hour and many calls with no resolution. Their position is that it is a LiveMail problem since I can access my RCN WebMail. I have read on other forums, some dealing with RCN, on this exact issue and they all say it is the ISP who can remove this block. Is RCN correct and I need to get LiveMail tech support to help me delete this message or is there some way I can access the sent email to delete the problematic email myself. None of the backed up sent emails or the one causing this issue is in my Sent Box and I see no other Outbox of any kind to access these sent emails. Basically I am in a recurring error loop with no exit. Any suggestions are deeply appreciated as RCN is no help.