JUST AN FYI and hoping to save some of you some grief- If you have an Netgear ORBI 3000 mesh network.
Was having issue where emails would not send but I could receive.
Noticed that recently after updating the ORBI to Firmware 2.7.5.6 and that is when I started having this issue. Long story short, I went back to previous Firmware 2.7.5.4 and problem went away.
In addition to emails not sending, I also had issue with PlutoTV losing connection every 10 minutes like clockwork and needing to just refresh the browser.