Apple Mail changing characters in Emails

I'm having this problem at work where emails instead of displaying the original message, are displaying repeated rows of "!" or "A" in it's place. We played around with the fonts but there isn't an option to change it.

It's set to Helvetica, some of the fonts stay the same for 60-90% of the emails, while some emails the whole message is all random characters. Even from the same sender. Sometimes a whole email will be fine when received and then when I press reply most of the messages are either "A" or "!" This issue only started happening within the last few weeks. I've posted some samples.

Is there a way to make sure that all the fonts are the same across the board? Or at least show up properly? Is it because I have lots of fonts on my computer? I have Message List font, Message Font and Fixed-Width font all set to the same font but only some parts of the message give me this issue. Is there a way I can reset it maybe? The only way I can see the original message sometimes is to paste into notes/text edit and change the fonts there. But again it happens 60%-90% of the message with the same font set for all parts so not sure what's going on?

First sample is of the Subject, From, Date etc. It's coming up as all "A"s

This is the same person's signature. The message however is fine.

This one is from a different sender and just their email signature is giving issues everything else is fine.

I can see the sender, date and time in the column next to the open emails, but for some reason this all happens and continues to happen in an email thread.

My coworker said it may be because the sender is using PC but this only just started happening.

Also, I know some people's computers are MACs.

I can change the font if I were to press reply and select the text and right click>font but that's a lot of work to answer emails. Also it's not every email. I feel like it may be people using computers, because some emails are fine and one of the people I talk to and this issue doesn't occur with uses their phone to communicate via email.

Sorry for the long detailed post but people usually give the "Have you tried turning it off/on" type of answer and details get that out of the way. Any advice helps!