“If you are using SMTP port 587 you set to Use STARTTLS (Always).”
That is the problem. I had been using STARTTLS(Always) for the last 6 months and today eMClient had issues sending email via 587 with that setting. Thunderbird had the same setting and it had no issue. eMClient repeatedly refused to send outgoing mail even after I shut down eMClient and started it back up.
Only when I changed the setting did the problem go away.