If you are unable to connect to your POP3 email using Gmail in Umango, there are a couple of things to check are enabled.
If captcha is enabled on your Gmail account (and it is enabled by default), then Gmail POP3 authentication will fail even though a correct login/password is provided
From the computer on which your logins fail, visit http://www.google.com/accounts/DisplayUnlockCaptcha
POP mail retrieval is disabled in Gmail by default. Follow the instructions found in the link below to enable POP3.
Also, ensure that Gmail's POP download deletion option is not set to 'Delete Gmail's Copy'.
POP mail will fail if your Google account has less secure apps disabled. Follow the instructions in the link below to ensure less secure apps can connect to Gmail.
If the setting seen in your account security settings (https://myaccount.google.com/security) under "Less Secure Apps" does not show as "On" then you will not be able to use Umango for Gmail POP retrieval.
If you are POPing Gmail in another application besides Umango, you may need to enabled 'recent' mode in order to retrieve email reliably. This is explained in the Google article below and is implemented by prepending your Gmail account name in Umango with 'recent:' (eg. your account would be entered as 'recent:myemail@gmail.com'. This will enable Umango to always retrieve files from Gmail.
Link to this article http://umango.com/KB?article=89