Hey Atakan, considering the exact same code worked in your previous environment, this has to be a setup issue, therefore looking for either unblocking the insecure port 25 or, even better, switched to port 587 would be recommended.
Hey Zsolt,thank you for reaching out to us, this particular use case isn't very well documented and will be added to the docs in the future.Instead of directly writing your header into the header field, write it like this:headers: {
custom: {
...
