You need to add any desired carriage returns explicitly, e.g. using the pilcrow character (which I don't have on this iPad, so let me use Char(13) instead).
Any returns that you add via keyboard will be blanks within a text string, snd disregarded if they are outside a string (so-called 'whitespace').
Try something like
cr = Char(13) ;
"Dear " & Table::nameLast & "," &
cr & cr &
"your e-mail body text" &
cr & cr &
"Best regards," & cr &
where the formatting has no impact on the actual result text, but helps to visualize it.
When I use the File>Send>Mail pathway, it sends it thru Mail.app without stopping for review or edits,
Fiddle with the 'dialog on/off 'setting; there is one that will open the mail in the client application without sending it.
If you want the email to go to the mail client, in the Email Script step, choose to Send Via "E-Mail Client" and it won't go straight out and will instead open in your mail client. But if you send via the "SMTP Server", then FileMaker has its way of dealing with characters which can be different than your email app or even other plugins. If you do a lot of emailing, using a plugin will most likely give you better performance and control over special characters and multiple attachments, etc. Actually, for things that don't need edited, I usually send HTML email with a plugin and format everything with HTML code which looks good and is more predictable as to how the things will display (e.g., using <br> and <p> codes to break and start new paragraphs). FileMaker's current email is a bit limited and made from old code. But they said in their Road map that it is one of the things they expect to re-write in the near future... and I hope it will be in 16. We'll just have to wait and see.