Dovecot server receives email instantly from Hotmail, but mail from Gmail never shows up

Question

I have set up Dovecot and it's working to both send and receive to my hotmail address, and the response is instant both ways.

However, I can send to Gmail (and Gmail gets it immediately), but not reply from that email, or send an email back which is received by my server. I have checked and re-checked in the /var/mail/vhosts/mydomain.com/myemail/ folder.

I do not receive a delivery failure notification.

What might be the cause of this? I have checked my IP's reputation at mxtoolbox.com, and there's insufficient information for senderscore.org. Is this maybe an SSL issue? Sorry, I don't know enough about this to use the right vocabulary but have managed to isolate the problem.

-- EDIT, ADDITIONAL INFO --

On further inspection in the dovecot logs I find this happens, so it looks like Gmail is sending but dovecot is discarding:

postfix/smtpd[15375]: warning: cannot get RSA certificate from file "/etc/ssl/certs/dovecot.pem": disabling TLS support
postfix/smtpd[15375]: warning: TLS library problem: error:02001002:system library:fopen:No such file or directory:bss_file.c:398:fopen('/etc/ssl/c$
postfix/smtpd[15375]: warning: TLS library problem: error:20074002:BIO routines:FILE_CTRL:system lib:bss_file.c:400:
postfix/smtpd[15375]: warning: TLS library problem: error:140DC002:SSL routines:SSL_CTX_use_certificate_chain_file:system lib:ssl_rsa.c:701:
postfix/smtpd[15375]: connect from mail-io0-f181.google.com[209.85.223.181]
postfix/smtpd[15375]: lost connection after STARTTLS from mail-io0-f181.google.com[209.85.223.181]
postfix/cleanup[15376]: 8E1087DDDB: message-id=<[e-mail]>
postfix/smtpd[15375]: disconnect from mail-io0-f181.google.com[209.85.223.181] ehlo=1 starttls=0/1 commands=1/2
postfix/qmgr[19456]: 8E1087DDDB: from=<[e-mail]>, size=908, nrcpt=1 (queue active)
dovecot: lmtp(15378): Connect from local
dovecot: auth: Debug: master in: USER#0111#[e-mail]#011service=lmtp
dovecot: auth-worker(15379): Debug: Loading modules from directory: /usr/lib/dovecot/modules/auth
dovecot: auth-worker(15379): Debug: Module loaded: /usr/lib/dovecot/modules/auth/libdriver_mysql.so
dovecot: auth-worker(15379): Debug: sql([e-mail]): query: SELECT email as user, password FROM virtual_users WHERE email='$
dovecot: auth-worker(15379): sql([e-mail]): unknown user
dovecot: auth: Debug: userdb out: NOTFOUND#0111
postfix/lmtp[15377]: 8E1087DDDB: to=<[e-mail]>, orig_to=<postmaster>, relay=myserver.com[private/dovecot-lmtp],$

dovecot: lmtp(15378): Disconnect from local: Successful quit
                                                                                                                                                   postfix/bounce[15380]: warning: 8E1087DDDB: undeliverable postmaster notification discarded
                                                                                                                                                   postfix/qmgr[19456]: 8E1087DDDB: removed

Answer 1

It looks to me like your postfix server is advertising STARTTLS and GMAIL is trying to use it but when postfix actually tries to enter SSL/TLS mode it fails to do so.

Googling your error turns up https://serverfault.com/questions/9485/how-to-fix-mail-server-ssl which may be helpful.