This has happened to me twice in the last few months and each time I had to open a new email, tell everyone about it, change email for my bank etc.

When I go to send (reply) email gets rejected and I get this error:

Technical details of permanent failure:
Message rejected. See Why was my message rejected? - Gmail Help for more information.

I tried emailing Google but never got any reply.

Why does this keep happening and how to fix it?

I don't even send those funny forwards, just normal emails..

Basically, the GMail automatic software thinks you're trying to send SPAM or otherwise inappropriate e-mail.

Are you sending e-mail to a large number of recipients?

I'd have to see an example of what you're trying to send in order to diagnose it further.

No I reply to a single person.. I work in sales so I do get a lot of emails but whenever I reply it's always to one single person.

When I get that error it prevents me from sending ALL emails, no matter what subjects, text, receiver etc.

Sometimes it goes away for a day or two then comes back again.. very annoying.