I wrote this on September 7, 2021 and published it on linkedin. However I found that it can be hard to search if I put there. so I put here anoter copy TLDR: if your external SMTP is using port 80 instead of standard ports in an istio mesh, create a Service Entry for the external SMTP. These two days, a strange timeout issue happened in one of our kubernetes clusters when trying to send emails via SMTP even though the same configuration works perfectly on our development machines.