Since my previous attempts failed I decided to go the default route using Fastmail IMAP and SMTP as advised on their web help page. I set my .muttrc file as follows, hoping it would be easier to resolve the problem.
set imap_user=ionm...@eml.cc set imap_pass="my fastmail password" set realname ="Ion" set from = "ionm...@eml.cc" set folder=imaps://imap.fastmail.com:993 set smtp_url ="smtp://smtp.fastmail.com:465" set smtp_pass = "my fastmail password" set ssl_force_tls =yes set header_cache = ~/.cache/mutt set spoolfile = "+INBOX" set record ="+Sent Items" set postponed="+Drafts" set trash = "+Trash" set editor ="nano" set timeout=15 set mail_check=1 set check_new=yes auto_view text/html But the response was... "SMTP session failed: read error"