Delivery attempted for sending
When you select a list for sending an email, Mail250 filter out any potential BAD email addresses or email addresses which have already received for which bounce/unsubscribe or abuse complaint have received in the past.
This filtration helps to protect the reputation of your sender domain without hitting the ISPs and anti-spam filters.
After suppressing these types of email addresses, Mail250 attempts to deliver the emails on the rest of the potential good email addresses.
This filtration doesn’t necessarily, mean that your list is not completely clean and email will for sure be delivered and not bounce. Filtration is just an attempt to protect your reputation and not to disturb end-users who are not interested in receiving your emails.