To apply for a job even though I don't meet the essential criteria?

[Stiltons]

I'd like to apply for a job in a different field from mine. I have a lot of skills that would transfer across but one of the essential criteria listed is a knowledge and experience of a certain type of software. I have never used anything like it before but I would certainly be capable of learning.

Is it worth at least approaching the person given as the informal contact? How would I word an email? 'Hi, I'd like to apply for your job but I don't have this thing you've asked for' seems a bit wet.

Just apply! In your cover letter mention that you don't have that requirement and tell them you are willing to learn (and maybe give an example of another skill you have managed to learn quickly).
I have interviewed people for a role where only one person met all the essential requirements, but 6 people were invited to interview. That person didn't get the job (for many reasons). Best to be honest.

You should definitely go for it.

One of the reasons that on average males tend to have a steeper career trajectory than women is that in situations like this, generally a male will have no hesitation in applying and a female won't apply - but actually employers don't really know what they want and something like software knowledge is simply not a barrier for employing the right person for the job in general. You can learn a software in a day or two, its not like an innate personality trait.

I applied for a job once where I needed a particular software knowledge (though I think it was strongly desirable rather than essential in the JD) and I put on the form that "I am familiar with and have a good working knowledge of xxxxx" despite never having used it but having watched the publicity video on the software company's website. When I got an interview I found a distant acquaintance who used the software professionally and got them to do me a half hour session touring the main features. Yes I got the job and it was fine.

You could appeal mumsnet for experienced users in your area if its likely to be complex enough to warrant that, but tbh that demo wasn't that vital. Its normal for any new employee to take a little time to find their feet and maybe have refresher training on something they haven't needed to do for a while. If you get the job you can upskill yourself then.

I got my first job when I called the listed contact after the applications had closed. Figured what could it hurt, worst they could say was no. Spoke to him for a good amount of time, told me to come on for an interview. Got the job.

So go for it would be my advice!

Go for it. Some things like a software package or a new process can be taught easily to the right person. Other things such as diplomacy, reliability and willingness to learn are either there or not. Good luck!