My boyfriend has long emails with other women, how can I accept that they're just friends?

Do you trust him? This is a touchy subject for both of you. Have you & he discussed these women & the emails? I don't know how you can accept that they're "just friends" unless you trust him, and i don't know if you and he have earned each others' trust. Has he given you other cause to doubt him? And how did you discover these emails -- maybe by reading when he wasn't there? Trust can be a very tender thing.
That's a problem. They may or may not be just friends. You have to be brutally honest to yourself about your relationship with your boyfriend. Is it what you want and is your boyfriend committed to it? Another issue is your reading his e-mails to these other women makes it seem like you are invading his privacy and are going behind his back. You might want to talk to him directly about your concerns.
Is he open? . Does he try to protect or hide these emails or is he comfortable with you reading them? I would be careful ...There is such thing as an emotional affair where the couple does not engage in sex but become dependent upon each other for emotional support. He should be looking to you for emotional support. Discuss these things with him...Good luck.
Trust&Understanding. While every relationship is different, one of the most important aspects of any romantic relationship is trust. Have the two of you talked about what is important to him about the e-mailing and what causes you to feel uncomfortable about it? It might be helpful for the two of you to talk things through with the help of a therapist.