What are send errors?
The send errors are errors your email server is giving back to Mailster basically saying "I will not forward this message". Mailster retries that for some time but eventually stops which is what you see happening.
The cause can be a lot of things, e.g. hitting send limits (per hour/day) or sending email with content that the server does not like.
How can I fix this?
You need to find out what your email servers are telling Mailster. For this enable the "DEBUG" logging mode temporarily (described in the article in the troubleshooting section).
Then reset the emails not being forwarded or have new messages processed by Mailster. Wait for the error numbers to increase. Then look in the email archive in the backend, open the email in question, switch to the "Send Report" tab and look through the messages there. This should give you an indication what is going on.