The fact that our own experiences so strongly shape our views comes through really clearly on this thread. The only thing we can know from the information shared here is that we can’t know anything for sure.
The view (from one poster in particular) that so-called ‘good’ men can be tempted/taken away by predatory single women is really counterintuitive to me. I suppose the first and obvious point is that humans can’t be easily divided into categories of ‘good’ and ‘bad’; we are all varying shades in between. I do understand that someone who has generally been a good partner otherwise could end up having an affair in the ‘right’ coalescence of circumstances. Nonetheless, men absolutely still have a choice there and that person could not then be described as being entirely ‘good’, if we view goodness as the sum total of someone’s actions. I think that the ‘good man tempted by a predatory single woman’ narrative is reductive, retrograde and damaging, setting women up to view one another as threats or competition, with the man as an inert penis-driven prize to be kept.
That said, if someone feels the need to intervene, I don’t think that’s necessarily as awful an indictment on the state of the relationship as some posters have suggested. Intervention could mean simply stating that you don’t feel comfortable with something as a means of opening up a discussion, while maintaining a questioning approach to your own feelings and where they stem from, which is less likely to make a partner feel policed or controlled. Or it could be a really heavy-handed and controlling approach which isn’t indicative of a healthy relationship. I don’t like the phrase ‘nip it in the bud’, however, because I think it positions women right back in that partner vs. other woman dynamic, a narrative in which the man to a greater or lesser degree is the unwitting victim of a siren and needs explicit re-direction to control their sexual appetite. This can also create the all too common parent/child dynamic where the woman as the ‘responsible’ adult has to educate the husband, or ‘child’. Personally, if I felt like I needed to take any significant action myself to nip something like this in the bud, I would be questioning my relationship. I would expect my husband to have the wherewithal to recognise his own feelings - whether objectively condemnable or otherwise - and carefully consider the consequences of any actions he might take either way, without my intervention. I am aware that not all men (or people) have that level of self-awareness of the connection between feelings and actions, but as a trait that would worry me in a partner.
OP, maybe there is nothing to worry about here. The message is too gushing for my taste, but this woman could just be like that with lots of people. Maybe she wanted to send a more personal message (hence the personal email) without the intrusiveness of a late night text because she has had previous struggles at work and appreciates this new working relationship. Or maybe she does want more from your husband, and he does or doesn’t reciprocate those feelings - no one on here can know. I hope that, through open discussion with your husband, you can reach a resolution.
Editing to be clear: I dislike the phrase ‘nip it in the bud’ as an action that the (in this scenario) woman has to take; no issues with the male partner in this situation taking such action if he himself views the message as an overstep/thinks there may be more to it.