I have been using a Motorola SB6141 modem with Asus RT-AC66U router for a few years and just started having only outbound email issues a few weeks ago. I have found the root cause of my issue, albeit through my own troubleshooting, since Comcast support is pretty useless...unless you want to try rebooting your router (and wouldn't help in this case anyway.)
In order to now get any outgoing message sent from my Outlook 365 mail client I must disconnect my VPN tunnel connection and then click the Send/Receive button in Outlook again after the VPN is off. Outgoing emails then get sent 100% of the time.
Apparently, the smtp.comcast.mail outgoing mail server has been updated or changed recently and no longer allow customers to send emails with an active VPN connection to the internet. Try disconnecting any VPN service that you have running and see if that fixes your sending mail issue.
This is not a permanent solution but provides me with a workaround to get outgoing emails working despite Comcast Support.
... View more