A pair of problems
 

Firstly, the ability to send "spoof" emails (using fake addresses, such as the obviously bogus snape@hogwarts.sch.uk) has disappeared. (This is a problem with the backend, not the new interface, as I can't do this from Pegasus Mail either.) This has prevented me testing the modified filters on my Guest account to see if they still work (I have set them to auto-forward to my Basic account).

The other, more serious, problem is that there doesn't seem to be any way to reply including the previous message as inline text any longer. The reply is auto-included as an attachment in .EML format, but this is not a good idea as (1) I suspect a lot of people won't be able to open this file, and may not even know what it is; (2) not including it inline defeats the purpose of including it at all; and (3) several email providers mistake this type of file for a binary which could be a vector for malicious code (it's actually a UNIX text file).

Is there any way around these problems, or at any rate the second one?

To rectify the second issue, go to Settings->(MAIL) Preferences, and click Show advanced preferences in the Writing section. Make sure Include the original message is enabled with quoted or inline in the drop down box to the right..

It works for me with an email client, I just sent an email with a made-up domain through mail.messagingengine.com using claws-mail. An alternative is to set up a temporary indentity in webmail.

BTW a .eml file isn't specifically a UNIX text file, it's a raw email in a native text file (i.e. a text file with native line terminators). I think the three letter file-name extention is actually from Microsoft. Strictly specking it isn't guaranteed to be text file since mime sections can contain non-encoded binaries.

Sorry to disagree, but that's a false statement. For years you have been able to create an identity such as *@fastmail.com using the wildcard * local-part. When you select that address at the top of the Compose screen, you will see a From field appear which can be freely edited to set the From address of the message.

Bill

As previously noted, it's easy to change the reply behavior to inline. I don't see the big problem if you do include a .eml attachment, since it has been a common format for saving email for many years. In particular, you can read the .eml attachment using:

• Fastmail web interface: Click View for that attachment and you will see a new window which shows the original email. My original test message had two .eml attachments, and I can read an .eml message inside a .eml message.
• Gmail web interface: The original .eml attachment message and sub-messages (.eml attachments as described above to the .eml main message) are automatically expanded inline.
• Yahoo web interface: The original message inside the .eml attachments is displayed inline automatically. Sub-messages are shown as attachments.
• AOL web interface: The .eml attachment message and sub-messages are automatically opened inline, similar to Gmail.
• Outlook.com web interface: I can read the .eml attachment main message and see the .eml sub-messages.
• Outlook email client: I can read the .eml attachment and the .eml sub-messages.
• Thunderbird email client: I can read the .eml attachment and the .eml sub-messages.

So the .eml attachment type is very commonly used in web interfaces for email providers and in email clients. Which popular systems don't handle .eml correctly?

Bill

Presumably "it" refers to email spoofing. I think the clue in the above is that you're using the deprecated mail.messagingengine.com server. My Basic account no longer has the ability to do this, and when I use smtp.fastmail.com (either through the web interface or through a client) I cannot use a predefined spoof address; it doesn't work. I haven't tried the way Bill (@n5bb) suggests, so I don't know if that still works.

I just tried spoofing again, this time using a wildcard identity as suggested, and it still didn't work. I suspect this facility has been removed from (or never implemented on) the new server.

Bill everyone could create any from address in the old UI, you just typed it in from the sent from address when sending an email.
That no longer works in current UI as we all know as a simple change of address you have to have a work round.

As we all should know, for many years it has continued to work as long as you use a wildcard identity as I described. It's the same feature -- just a different way of using it. It never went away.

It still works for my Basic account (and other accounts). In addition to the web interface method working, I can also use an email client (Thunderbird) to send with a random From address through the newer smtp.fastmail.com outgoing server (using the Basic account SMTP login).

Of course, it's quite possible that this feature is being removed for security purposes. I would be interested in knowing if any other people see this failure. It has always worked for the old (mail.messagingengine.com) and current (smtp.fastmail.com) SMTP servers.

Bill

For whatever it's worth, I just tested this myself using a completely fabricated address and it worked just fine when sending through smtp.fastmail.com.

As Bill says, it's possible this is being deprecated, although I've seen no indications of that. Another slight possibility I can see is that perhaps Basic accounts don't have the ability to do this — since they can't use custom domains, there'd be less of a technical reason to allow custom from addresses via SMTP clients. However, Bill notes that it works with his Basic account, and personally I can't see FastMail making that kind of a distinction.

So I'm not sure what's different in the OP's case. I'd suggest making sure that your third-party SMTP client is properly authenticating to FastMail's servers with the proper username and password, as there will naturally be more restrictions on what you can send from an unauthenticated connection.

Also, just to clarify, when you say it doesn't work, what is the result you're seeing? Is the message being rejected as soon as you try to send it (e.g. at the SMTP level), are you getting some kind of bounce-back, or does the "spoofed" message simply never arrive at its destination?

Okay, just to take it a step further, I tested the example address that the OP provided (snape@hogwarts.sch.uk) both from Apple Mail sent through smtp.fastmail.com and the FastMail web interface (by using a wildcard address and typing it in exactly as shown above). It worked completely fine in both cases, with the "spoofed" message arriving at all of the three destination addresses I tested it with.

I think what the OP was trying to say it was easier to do in the old ui as all you had to do was type in the outgoing address, that's all.

While that's obviously part of it, which Bill already addressed a couple of times by explaining the use of a wildcard Identity in the new U.I., that's not entirely the problem the OP is having, since they said:

Meaning it's not working with third-party e-mail clients either (Pegasus Mail in this case). They later noted that they tried it again using a wildcard identity, and it still failed. So either they're doing something different from Bill and I are, or there's some problem specific to their FastMail account.

I just tried in Thunderbird and changed all the from addresses in the account settings but it still gives out my fastmail default ID but does show a fake return address.