My boyfriend talks a lot with my best friend...

I think we can't say anything definitive without talking to your boyfriend, but let me just say that sometimes we don't think about things that might make girls jealous. If it's in his mind as something 'insignificant,' he probably isn't doing it to hurt you. However, if the frequency of their communication bothers you, gently bring it up; he might not even be aware of it.

To be frank, in my neighborhood, such situations have always escalated into bad places. No one tolerates too much intimacy with your closest friend. I mean, if he’s your closest friend, he should also pay a bit of attention to the boundaries of the relationship; I think guys don’t tend to pay attention to this, but your close friend should also show a little care. You need to sit down and have a serious talk with both of them; this isn't something that can progress quietly.

It seems to me that you're being a bit too sensitive. I mean, think about the same sentence in reverse; if you were messaging your boyfriend's close friend, would you blow it up this much? If you're just talking about 'unimportant things,' it might be turning into a trust issue for you. If you trust both of them, I think you shouldn't pick at it so much; ask yourself a bit too, why are you feeling so jealous?

I'll make an update; I haven't noticed anything problematic in the correspondence. There's no change in her attitude towards me either, but the closeness of their friendship bothers me. You know what they say, a woman knows another woman very well — I might not have understood my best friend's intentions, but I don't feel completely at ease. I'm hesitant to bring it up because I think I'll feel worse if my boyfriend says 'you're being ridiculous.'

Now, in situations like this, the ambiguity of boundaries usually causes problems. Is writing too much to your closest friend a need, or is it just a habit? Instead of saying "I feel uncomfortable" while talking to your partner, it would be better to explain what bothers you in the frequency of your conversations. Don't confront directly; starting an open dialogue in a tone like "you're special, but I'm jealous" is less likely to put them on the defensive.