Keep in mind that if he were having an affair, it is not likely the woman would send a cheap card saying I love you sexy on it, whether she was a serious love interest or someone who had affair but he ended it, because the former wouldn’t do something that dumb looking, and the latter wouldn’t seek revenge by saying she loves him and thinks he’s sexy.
Also, the lack of a name points to someone who is not a love interest, does not have an intimate bond with your husband, and doesn’t want to get in trouble or associated with placing a name on envelope.
So, I’d say it is a third party who wants to cause trouble. You’ll have to do some digging to find out if it has any basis in anything untoward your husband’s done, or if someone is angry at you or him for some unhinged reason.
It sounds like something a teenager or teenagers would do for a prank. But there will some kind of weird reason behind it.
Have you or your husband celebrated a success or happiness recently? An enemy gets really mad when a person is happy or possesses something (tangible or intangible) that the enemy doesn’t have and wants.
If it is an affair, it will be a third party who knows about and has sent the card.
Are you on social media? The envelope was posted from your old hometown. If someone saw you post something happy, they may have sent this in spite. Do you have frenemies from there? Did you mention h’s upcoming birthday on social media?