I have a V2760 on a basic home network and use an attached USB disk to capture Syslog files. After updating firmware to 3.8.7 (was previously 3.7.8), the occasional Mail Alert emails advising of DOS attacks stopped. The update also coincided with the stopping of receipt of Web Syslog files several times a day via email.

After trying different configurations of email accounts and SMTP port numbers in System Maintenance: Syslog/Mail Alert to no avail, I gave up and raised a support ticket. The upshot of this was
• to confirm that Mail Alert did still work on 3.8.7 and
• to advise me to follow the path of using a Mail Service Notification Object as outlined here https://www.draytek.co.uk/support/guides/kb-mailnotification

After getting erratic and inconsistent results from the Notification Object, I have dug into the weeds of SMTP and checked the behaviour of the different email providers and accounts that I use via telnet and MXtoolbox.com. I have been able to get both the Mail Service Notification Object ad the "Send a test email” button on the System Maintenance: Syslog/Mail Alert screen to work.

According to the requirements of your email provider you may need to enable SSL and/or authentication, but for me the key to success appeared to be that if any change is made to the email parameters, it is necessary to re-enter the authentication password (even though it has not been changed) and then OK the changes.
I have not yet managed to restore operation of the regular Web Syslog emails.
I hope that others may find this useful