; require 'iso-acc before erc to avoid following breakage:
;Signaling: (invalid-argument "Unknown keysym specifier" #<keypress-event s>)
;  character-to-event((#<keypress-event s>))
;  iso-char-list-to-event((#<keypress-event s>))
;  iso-accents-compose(nil)
;  iso-accents-accent-key(nil)
(require 'iso-acc)

Inside
(custom-set-variables ...)
I have
 '(mail-setup-hook '(iso-accents-mode)
)
and
 '(message-setup-hook '(iso-accents-mode bbdb-insinuate-sendmail bbdb-define-all-aliases flyspell-mode)