ot: LE server conf setup/ iPhone 'expired cert' message

I've installed LE certs on my Dovecot a while back, and, it has been working OK since, but, today, an iPhone user said he can't get emails as iphone says 'cert is expired', searching around, I see some other iPhone similar issues reported, do I have my conf correct, I have;

# cat dovecot.conf | grep ssl ssl = required verbose_ssl = no

ssl_cert =

is fullchain.pem and privkey.pem is what I should be using ?

anythought how to force an iphone to reload cert ?

actual cert was renewed 15/7, old/previous one expired earlier today

ls /etc/letsencrypt/live/fqn.myserver/ cert.pem chain.pem fullchain.pem privkey.pem

(if I open mailserver host in browser, padlock shows current/valid cert)

-- Voytek