Let's get a few more details.
Are you / your household a TalkTalk Consumer broadband customer?
What's the domain of the email address - the last bit after the @
If the mailbox is on the TalkTalk Mail platform please sign in to TalkTalk Mail. Tell us if there's an error message or if you can sign in ok using the email password and then send the same email address a message to make sure the mailbox can send and receive messages.
Please confirm if it's the gMail app that you're using on the mobile phone.