. . . my husband's girlfriend called me a selfish bitch in an email to him, and was almost as insulting to me directly.
Egad!
We all can behave selfishly sometimes and not realize it until others point it out, but name calling serves no purpose - or at the very least, doing so gets in the way of the message. There have been times that I've said to people, "You are being selfish," and they've responded with, "You're right, thank you" because they were able to hear what I was saying since I didn't say, "You're a selfish brat/bitch/motherfucker/whatever." The times I have name-called, I almost always regretted. It goes both ways - I can "hear" when people call me on my shit if it is expressed directly and matter-of-factly, and not hurled at me in an insult. It certainly doesn't feel good to be on the receiving end of it, unless it's someone whose opinion really doesn't matter to us, but I would hate it if a partner of mine didn't put an end to that kind of shit right away and ask that I be spoken about respectfully or not at all.
I don't have a lot of experience with having a metamour, but when I was seeing Burnsy there were some issues I had about his wife. I thought she sounded a bit selfish in some ways, actually, but I had only had limited contact with her myself and had to go on what he told me about her to get a sense of who she was. This was an LDR and I'd never met her in person. STILL, I always felt that I should never say anything against her to him, and that if I felt the need to address something that bothered me about how she treated him (as far as what I was told by him), I knew I could, and would, address it with her directly - but never by calling her a bitch, certainly!
If I ever wanted to express certain things to him about the way they were "doing poly" or how it seemed he was being treated by her, I focused on HIM. In other words, I would ask, "You seem dissatisfied with that," or "How do you feel about such-and-such?" and "What do YOU want, Burnsy?" What I said to him when I ended it: "I feel that you need to come to terms within yourself with how you feel about [certain issues] in your relationship with your wife, before you can really fully be with me." I could only address what I saw in HIM, and how he expressed his relationship, but I never, ever, put her down in any way.
If your husband's gf sees things when she is with him that she perceives as bothering him, problematic in some way, or whatever, she needs to focus on HIS process in her inquiries or conversations about it, not blame you or call you names, as not only is that immature (sheesh, and a PhD, too), it accomplishes nothing. Besides that, it isn't her place to comment on his relationship with you. She should only worry about his relationship with her. IMHO.
However, consider this -- many people do have a hard time delivering difficult communications. Maybe she is having an issue with something that, although it could have been expressed much more diplomatically, might be good for you to take a look at and discuss with her and/or your hubs. Maybe there is something you do that she is interpreting as selfish or maybe there is some selfishness in you, regarding a certain issue, that you aren't aware of or impacts her somehow. As they say, don't shoot the messenger. Just see if there is any validity to what she said, talk to her about why she feels that way, and ask her to deliver her communications about you directly to you -- and in a less caustic way.