Sounds more like a DNS failure than an SMTP issue.
Check the date and time as well as the DNS setup, it is almost certainly in those 2 places you will find the problem.
I have 2 WC 3655s, just one of them is presenting problems with scan to email option. I think that I already tried everything to solved it, but it keep showing the error message "Invalid Hostname" For example:
What could be the problem? Network Card or something from hardware??? Is the software from device corrupted?
Complete Error Message: "Job error. No message is sent. Network configuration error device (invalid host name). Contact your System Administrator."