Sorry for the delayed response was busy with making new releases of mojoPortal and other features and just getting through all my backlog of messages.
"System.Net.Mail.SmtpException: Mailbox unavailable. The server response was: 5.7.1 Client does not have permissions to send as this sender"
Make sure that the default "Email From Address" in Site Settings is a valid email for your mail server, and since this specific email is from Event Calendar Pro, make sure the "Email From Address" in Settings > Ticket/Registration Settings is either blank (in which case it will use the one from Site Settings) or a valid email address that can send mail from your domain
Note that <appSettings in user.config override ones in Web.config so for any setting that does exist in user.config the same setting in Web.config does not matter, it will use the one from user.config. If it doesn't exist in user.config then it will use the one from Web.config.
Normally I would expect that a username (or email address) and password would be needed in the smtp settings ie SMTPUser would be an email address (typically the same one as in Site Settings) and SMTPPassword would be the password for the given email address to allow it to send mail.
Hope that helps,