Skulls are being displayed in my email address instead of the @ symbol

Try hitting ⌥⌘U and locate the full From header. The skull is shown when the name part of a sender contains an email address which is not the same as the email address of the same header. This might be an attempt to spoof the sending address and therefore MailMate replaces the @ with a skull (to show the user that there's something unusual/suspicious about the sender name/address).

Do you know why this happens for these emails? Maybe there's a workaround which does not involve disabling the “warning”.

Right now (1.10 (5443)) it seems @ are being replaced by skulls also in strings such as Name @ Company, which to me is a bug… the pattern matching should only target email addresses shouldn't it?

Well, it's not a bug (it's intentional), but there might be a better way to handle it. Note that the skulls are an attempt to make sure that the user notices if the name part of a sender is an attempt to “hide” the address part. Some users might not notice if it's subtle, e.g.:

"good @ example.com" <bad@example.com>

MailMate would just display good @ example.com and the user might think that this is the real sender (despite the spaces). The skull is more easily noticed.

(It's a horrible idea to use Name @ Company in my opinion, but that's a different issue.)

There's a hidden preference available to completely disable the skulls:

defaults write com.freron.MailMate MmNospoofSymbolDisabled -bool YES

Thanks for the extra details.

If the intent is to make sure the user notices an attempt to hide the actual email address, do we agree that putting a skull in Name @ Company is a bug as long as Company isn't formatted like a domain name?

I like the skull idea so I'm unwilling to disable it, it just covers more cases than it should IMO.

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This doesn't look good when there are multiple mailbox-adresses in the From: field as described under "3.6.2. Originator Fields" in https://www.rfc-editor.org/rfc/rfc5322.txt (and 2822/822).

I like this idea as long as I can whitelist some contacts.

I noticed that newer builds now use an exclamation point rather than a skull. That's definitely better - but again, I would like to be able to whitelist some senders.