Amongst work collegues the boundries should be formed by the recipient. They are the ones who can allow the attention to go as far or as short as they want. A typical case would be if someone sent you an email at work saying you had "nice tits". If you felt this made you feel uncomfortable you should say so and then it is the sender's responsibility to cease that particular behaviour. Should they send further emails with sexual references this could then, by definition, be classed as harassment.
An understanding workplace might attempt some development in that area for those involved and perhaps issue a written warning for the sender. A more proactive employer would educate employees from the start of their employment but this is still a sensitive area for employers as this can be interpreted as patronising by some.
Unless the harassement was physical the matter would not normally be taken to the police. Dismissal is usually the last resort and the official reasons are rarely for harassment but for other work orientated reasons on which the employer can more safely base a dismissal without legal reprisals.
VFE.