[jhollington]

Okay, it sounds like you've got everything set up the way it should be.... I suspect the issue is about having multiple accounts configured....

Originally Posted by camner

Yes, though this seems to be happening by default. I just recently set up several new identities and I did NOT manually check that box! Perhaps that's the default action?

It could be, or more to the point I suspect it just copies the setting from your primary identity, as it doesn't get checked for me when I setup new identities (I use the IMAP save method).

I think the issue for me is that I don't really know how to tell iOS which mail account to use when using the sharing tools. I have several accounts set up in Mail; how does iOS determine which one to use when sending email via the sharing tools?

When you're sending a messaging using the built-in Mail app, whether from the app itself or the sharing tools, tapping on the "Cc/Bcc, From:" line should expand it into three distinct fields, and then you can tap on the "From" field to select the account from which you want to send the message. The only trick is that they're identified by email address so this will be confusing if you have more than one account configured for the same address.

Also, since outgoing mail is handled by an SMTP server, (1) how does iOS determine which to use when using sharing tools

iOS will use whichever SMTP server is configured as the "Outgoing Mail Server" for the Mail account/identity that you select to send the message (as described above).

since SMTP doesn't have anything to do with IMAP (AFAIK), how exactly would "let IMAP do its thing" actually result in a copy of the email ending up in the Sent folder of Fastmail?

If you specify your "Sent Mailbox" to map to your "Sent Items" folder under "On the Server" (e.g. on FastMail), than the iOS Mail client will automatically file a copy of the sent message into your "Sent Items" folder using an IMAP connection, even when using the iOS share sheet; there's fundamentally no real difference between going into the iOS Mail app and sending a message and using the share sheet — it brings up the same "New message" dialog and the same background processes are fired off to send the message.

To consider it another way, the iOS Mail app doesn't really treat IMAP and SMTP accounts as separate entities — you basically configure "Mail" accounts, which are a combination of IMAP/POP for retrieving messages and SMTP for sending messages, bundled into a single "Mail" account that is identified by the "From" address associated with that account.

There's no way to set up just an SMTP server in iOS Mail without it being associated with an IMAP/POP account (note that you can have multiple SMTP servers for a single IMAP/POP account for fallback purposes, but you can't setup a "send only" account that just has an SMTP server and no associated IMAP/POP server).

So in other words, when you select the "From" address of "you@yourdomain.com" in the iOS "new message" dialog, you'll be sending though whatever SMTP server is associated with that account (under Settings, Mail), and you'll be filing a copy of that sent message in whatever folder is specified for "Sent Messages" for that account — either a local folder or a server-based folder, according to whatever you've specified.