Is the username is entered correctly for the environment?
username
computer name\username
If the sender is a _domain_ user, specify the domain name
domain name\username
username@domain
Is the password entered correctly?
Enter _just_ the share name in the PATH field in the Address Book entry
Confirm the FTP path
Confirm the Windows NTFS permissions
Confirm the _share_ permissions
Confirm the FTP permissions
The following characters are prohibited
` ~ ! @ # $ ^ & * ( ) = + [ ] { } \ | ; : ' " < > / ?
Some (DNS) systems do not process FQDN’s that contain underscores _ correctly
If you are using a Gmail address for the first time with a device then Google Gmail may have blocked access for the device to log in even though the device is able to resolve smtp.gmail.com
Log into the Gmail account and turn on Allow less secure apps