Recently I have setup Meraki MX 64 in my network. Everything seems to be working fine but Scan to Email through Digital sender is not working i.e it gives Gateway Error. What could be the problem if someone could help? Do I need some smtp configuration in meraki dashboard? Please note i don't have any physical mail server as we are on DA and O365. Previously I was using smtp of ISP which was working fine before but not after this meraki setup.
By default, the MX doesn't have any rules blocking against outgoing. Assuming the SMTP you are trying to relay through is external there is no reason that should be blocked.
But a good practice, if O365 supports it, is to setup your relay through O365. In our G Suite environment, I have it setup as an SMTP relay with and it just whitelists my networks WAN IP. That way I can relay through our own e-mail environment rather than relying on a carriers SMTP. This also becomes important if you ever start looking into SPF, DKIM or DMARC.
I agree with Adam as far as using O365 to send your emails. This is possible to do but you may have some hurdles to jump through depending on your O365 security settings.
As far as testing the ISP SMTP server. Have you attempted to telnet to the server on the designated port to see if the connection can be established. If that doesn't work from behind the firewall, test it again with your laptop connected directly to the internet connection.
I have seen some "digital senders" break this year because they only support TLSv1.0 and Microsoft have dropped support for this old weak version of TLS.
You should see if there is a firmware update available for your unit, and make sure it supports modern strong protocols like TLSv1.2.