the default E-mail solution in my phone (E71) stopped working [with Gmail]
In your case, the certificates do not help. You would need that SIS. Since July 2015, Google and Gmail switched to SHA-2(56) and disconnected millions of devices, although alternatives like ‘ECDSA/SHA-2 + RSA/SHA-1’ do exist. The Nokia E71 is based on Symbian/S60 Feature Pack 1 and does not support SHA-2 or this SIS. I am not aware of any workaround except to
A) use a proxy based Internet browser, like Opera Mini
B) update to a newer phone – at least with Symbian/S60 3rd Edition Feature Pack 2 or newer, like the Nokia E72 or Nokia E6
If you use additional E-mail providers which do not work anymore either, please, name them. Then, I double-check whether it is the same cause. If the cause is different, there could be a workaround.
I have installed Opera Mini on my Nokia E71. I now use it to access my gmail account, which works properly. Thanks a lot for your tip.