Anti-virus ...

None. Every one of them is entirely worthless. There are no Mac viruses in existence, and not a single AV title can, or will even try to stop you from installing a Trojan.

AV software is even more useless for iOS. It's a locked down system. Due to sandboxing requirements of any app in the App Store, the only area an app is allowed to touch is itself. Meaning, the only place AV software in iOS can look is within its own application package. In order to do - something - and make it appear its doing anything of value, iOS AV software does completely useless stuff like deleting your browser cache. That can easily be done by the user in Settings.

The one and only exception is MalwareBytes for Mac. Its main function is to find, and then offer to remove adware and malware you've already installed. Not prevent it.

Your ISP has no idea what they're doing.

It is possible someone hacked your account on their end and was using it to pour tons of spam out that way. Just a guess though as to why they would tell you that.

It is entirely possible that you received a piece of malware and forwarded it on in an email. The malware had no effect on your Mac because it can't have any effect. But I agree with Kurt, it is most likely them misinterpreting something or getting a false positive.

Effective defenses against malware and ot… - Apple Community

You may want to peruse this as well.

Avoid phishing emails, fake 'virus' alerts, phony support calls, and other scams - Apple Support

I really need to make a quick access link for myself to that article.

It is a good article and sums up protection pretty well. But you did beat me by more than an hour so you really anwered the question, and you did it in detail.