Technically, the EU Data Retention Directive requires retention of comms data pertaining to 'Internet E-mail' - it doesn't make a distinction between SMTP/POP3 e-mail and web-based e-mail.

If an ISP is running a mail system for its customers, then it should have comms data from use of its own mail system. For webmail, it should be the organisation running the webmail system which retains this data & provides it to the police on request - as the ISP obviously knows nothing about this without digging into all the traffic its customers pass over the network. Of course, many webmail systems are outside the jurisdiction of the EU - which causes a bit of a problem!

Whether this is a good thing or bad thing is an interesting debate & I think less obvious than the case made by privacy advocates tends to state. The police have relied on such comms data from telephone systems for decades to help catch the bad guys ...