Positive word for 'unsolicited' - to have come out of the blue

"An unsolicited email" always sounds negative; "unsolicited" sounds negative.

But I would like a neutral or positive word that means that the email was unexpected/unasked-for, but not unpleasant.

If something were to "come out of the blue" for example, there are no implications as to whether this thing is negative or positive (in fact it might be slightly more likely to be positive).

This is the kind of feeling I would like with the alternative word to unsolicited email.

Thanks for reading this _____ email