Solution for WHM/cPanel(s) : Disable SMTP Restriction by following below process:
a) Open WHM and search for SMTP restriction, make sure it's disable.(You can go through Home »Security Center »SMTP Restrictions directly as well)

b) Or Same thing can be done via Tweak Settings (Directly go for Home »Server Configuration »Tweak Settings or you can click on tweak setting link shown in upper image)