Our system automatically detects most failed senders, however, every now and again it also sees perfectly valid email addresses as a fail, learn more..
Often people mispell their email, or more often, spammers enter junk posts. It's important that our system automatically detect any bad email addresses, and prevent further attempts to post emails to them. This also reduces the size of your active mailing list ensuring you don't waste the limits of your hosting plan.
Sometimes perfectly valid email addresses are detected as invalid. This is often due to certain spam filtering techniques. Unfortunately those specific mail servers deliver a "mailbox not found" error code, and we treat it the same as all the others. We are constantly tweaking our algorithm to detect exceptions to the rules, but it is a constantly evolving game. We are very dependent on the error codes returned by other mail servers.
You should check your failed senders report prior to sending newsletters, and consider if any important customers have been detected as spam. The report will list people in the recent order in which they were detected as spam, so normally you only need to check the first 10-20 emails, paying close attention to the "reason for the error"
You can upgrade a members status to undo their failed email status.
Nnot all mailservers return an error, and it is quite possibly that an email address is invalid, even though we have not marked it as so. Another way to deal with these sorts of emails are to pay attention to the "stale email addresses" report, and perhaps delete any stale email addresses that look a little invalid.