Big companies generally don't give out their email addresses to the public. That's a security measure to prevent their servers being swamped with emails in a 'denial of service' attack.
However (as T-Mobile is now part of EE) a bit of web searching suggests that an email sent to
[email protected] should get through.
Otherwise most companies use web forms and/or 'live chat' as ways of contacting them. EE doesn't seem to use web forms. However, if your query relates to a technical issue (rather than to, say, billing) there's a 'live chat' facility here:
https://ee.co.uk/help/get-in-touch
(NB: Select 'EE', not T-Mobile, and then use the button labelled 'Click For Technical Support')