Strange spam

Normally I receive little spam with Mac and my email-settings send them to the spamfolder. Unfortunately this works only on my iMac, not iPhone or Ipad. Normally it is no problem to delete them. But since a week I receive dozens of strange spam which scare me. They consist just of senseless combinations of letters like xnwbpa or olbtby, and the sender is such a combination of letters as well. The adress is not mine but different adresses unknown to me. As with all spam I delete them immediately, but first it is annoying and not what I am used to with Mac and secondly I ask myself, what is the purpose of them. Few have a HMTL, which I do not open of course, but most are without any appendix to open. So why do I receive them? Are they a method to infect my computer? What can I do to get rid of them?
Thank you for your advice.

With spam you are best off adding it to junk at source. So if your email provider has spam filtering you need to be marking it as junk using their system rather than just deleting it. This way it'll filter similar spam as time goes on.

Who is your email provider? Does it have a web login for your mail?

Thank you, mrplow,

my provider is t-online and I already did what you recommended and went to the web login and marked all these emails as spam. Since then these spam mails appear only in my spam folder, but that applies only to my iMac. Since iPad and iPhone don`t have that I still receive them there.

What still scares me: With spam people try to sell you something, want to steal your data etc. But what purpose do these mails have? Do they want to get on my nerves so much, that in the end I open one of the HMTLs which few of them contain and then they can plant something? Am I sure that they cannot harm me unless I open an appendix?

Marking them as spam online is the best way for the system to learn which in turn, will cut down the junk you get on all platforms.

Your precaution of not opening them is the best way. It's a bit of fuss but mark them as spam online and then delete any that make it through to your other devices. Your current caution is good practice