Gmail: Have you logged on to your gmail webmail account and selected either Pop or IMAP forwarding? In Thunderbird, are you creating the same type of mail account Pop or IMAP that you selected in webmail? make sure the email address does not have any typos. Make sure you are using the correct password. Generally, this password is the same one you use to access gmail webmail account. The reason may be to do with gmail who have made some changes regarding what they call 'Less secure apps'. Many people use more than a computer eg: phone to access their emails. Gmail created a two step verification for people who use eg: phones and this meant that anyone who had created this two step verification had to use an 'application specific password'. So if you use a phone and you set up 2 step verification, then you have to create an 'application specific password'. However, if you do not use a phone and just use computer, then you do not need the 2 step verification nor 'application specific password', but you may need to logon to your webmail account using a browser and select to use 'less secure apps'. See the info at this link: * https://support.google.com/accounts/answer/6010255?hl=en