Settings on spam filter for email-wanting to use Gmail

I use squirrel mail for my email but have been very unhappy with it. At first, there was no spam filter so I was betting bombarded. Now the spam filter is so strong that people keep being asked to verify that they are human every time they try to send me an email, even if they’ve sent something to me before.

My first question is, is there a way to modify the spam filter so that it doesn’t do that?

I would really prefer to run my wordpress.org site email through gmail and started to do the app for it but was told I would lose all my previous emails. Is this true? Is there a way to prevent it?