[UPDATE: The issue fixed itself. I am thinking the relay server which in this case Sendgrid had something to do with this.]
Everything was running smoothly until the Dovecot version got updated to 2.3.8 on my CentOS 8 server or at least that's when I start noticing it. Inbound mails are fine and transactional mails from my WordPress site are also sending. I am using Postfix, Dovecot and virtual mailboxes via PostfixAdmin. These are the content of the maillog:
Jun 30 13:34:10 bluesky postfix/smtp: error: open /etc/postfix/smtp_header_checkssmtp_address_preference: No such file or directory Jun 30 13:34:10 bluesky postfix/smtp: fatal: open dictionary: expecting "type:name" form instead of "=" Jun 30 13:34:11 bluesky postfix/master: warning: process /usr/libexec/postfix/smtp pid 950050 exit status 1 Jun 30 13:34:11 bluesky postfix/master: warning: /usr/libexec/postfix/smtp: bad command startup -- throttling
The only change I did since the Dovecot update was the ssl_dh parameter since it complained about the length and about not supporting SSL - if this has something to do with the issue...