Update user_manual.rst
Update user_manual.rst

The IP address for SMTP/API listed on line 228 was incorrect. Changed it from 198.61.253.48 to 50.56.21.178.
xjzhou/documentation,mailgun/documentation,stevenmc/documentation,mailgun/documentation,kaustavha/documentation,kaustavha/documentation,stevenmc/documentation,mailgun/documentation,stevenmc/documentation,xjzhou/documentation,kaustavha/documentation,xjzhou/documentation