I get a lot of duplicate spam emails but from different actual senders. The ones I’m curious about purport to be from CarShield. The subject line appears as "Important Message For You – Open Now! in a different font from other emails, so it stands out in the queue. But when I click on the email, the subject line changes to this: 55&5)5(5+5-55’5- 5 5 5,5,55 5 55(5+ 55(5. - 55)5 5’ 5 5(50! (there may be more but that’s all that shows on the screen).
Eventually somehow it changes back to the text, if I leave the page for a while, but it seems very random. Anyway, I assume this oddity gets some benefit for the spammer, and I’m just curious what they get out of it.
I would guess the subject line is not actually ASCII text but some other characters that happen to look like ASCII characters (similar to an IDN homograph attack). For example, maybe “Important” actually begins with a Greek capital iota followed by a Cyrillic lowercase te, etc. This would explain why it appears to you to be in a “different font”. It’s not actually a different font, it is non-English characters. Some parts of your mailer program may not handle the encoding of non-ASCII characters correctly and therefore display such text as the garbled string you see.
The reason spammers do this is to get around filters that are looking for specific keywords. So if a spam filter is looking for the string “Important Message For You” in the subject, it won’t notice the iota-te-whatever string that is actually completely different but visually looks similar.