Welcome to our forum, I am sorry you encountered difficulties sending email, we will do our best to help. VM501 is a message that indicates the recipient count has been exceeded in the last rolling hour, VM502 in the last rolling 24hrs. We limit the number of recipients in emails that may be sent to deter spamming. The actual limit depends on where you are sending from.
If we examine the logs for the same email address you have registered on the forum for Dec 5th we can see quite a few VM501 occurrences. These look to be genuinely applied as the sending recipient limits were reached for email sent from UK non virgin media connections.
If you wish to send to more recipients please either send from your Virgin Media connection or consider using webmail as the risk of spamming decreases so the limits increase.
We are currently implementing some changes to help with rejecting incoming email to our platform Spam Folder Settings Change so fingers crossed it helps with emails not reaching you.