Beware, you are facing serious dangers!
It is not about your personal anger (which is reasonable), there is a contact between two companies, both are important to your bosses and you can't see all of the circumstances!
For example, it is possible that your company mis-uses currently an old company relation on a way, that it causes trouble to the employer of your supplier company with it your are in contact! But this Bob doesn't know who are you inside your company, but he is angry to the company.
But there are a nearly infinite possibilities here. You don't know all details, but you know that you won't interfere with them.
Your primary goal is to avoid anything what could worsen the relation between your employer and the supplier!
And, of course, to protect your ass.
And, to show your bosses that you are wanting to solve the problems and relations.
If the bosses start to investigate, who insulted who, you have to make sure that nobody will find that also you made an offense.
Furthermore, Bob surely committed now a mistake. Particularly in the UK, where the customs are very polite. But you can see here also in the SE, full with people from anglo-saxon culture, that you can politely, indirectly f*off nearly everybody into the hell (and get the same back), but in the moment that you use any bad word, you will be the only badass in the whole picture on the spot.
So, what you should do:
- Simply ignore any insulting part of Bob's mail.
- React to the constructive part on your best.
- Not only try to defend yourself, try to really make Bob more friendly.
- If he repeats the offense, it will more clear that he is the bad man.
- If he doesn't repeat, then a later investigation will find that you solved an intra-company problem.
If the event doesn't repeat, it is a good thing to not mentioning the case upwardly. Maybe only as a nuance, and only verbally. ("Heyy, this Bob wasn't very happy, he wrote something that we are again late with the payment, but finally we could agree that the pallets will be delivered to the Liverpool garage until next Wednesday").
In general, a cooperative person hesites to transmit negative communication, tries to avoid it, while he is eager to transmit positive things.
And you want to be a cooperative person, with high social skills.
Particularly if you want to be useful for your employer in the communication and organizing tasks with partner companies.
Your goal shouldn't be to let exterminate Bob by Alice, your goal should be to take care to the good contact between the companies. And you want to show that you can solve communication clashes alone with good results.
Btw, if Bob doesn't do this repeatedly, then you should really look over it. Everybody can have a bad day, it is not a reason to conspire against him.