Yes, it's STARTTLS that the SMTP server is objecting to. The fact that it doesn't advertise STARTTLS in its response to the EHLO is a good indication that it doesn't support it.
This suggests that you need to tell the Drobo not to try using a SSL secured SMTP connection (but if you're not using authentication I wouldn't have thought you would be). If there's a "use ssl" tick box try unticking it.
Does sound like a broken SMTP client implementation (especially given the response from Drobo support).
# Which domains will this server relay
relay_domains = $mydestination, mysql:/etc/postfix/mysql/relay_domains
# Which domains/accounts will this server store
virtual_alias_domains = mysql:/etc/postfix/mysql/virtual_alias_domains
virtual_alias_maps = mysql:/etc/postfix/mysql/virtual_alias_maps
virtual_mailbox_domains = mysql:/etc/postfix/mysql/virtual_mailbox_domains
virtual_mailbox_maps = mysql:/etc/postfix/mysql/virtual_mailbox_maps
virtual_mailbox_base = /var/mail/virtual
virtual_uid_maps = static:1000
virtual_gid_maps = static:1000
# Which transport to use for each domain
transport_maps = mysql:/etc/postfix/mysql/transport_maps
/etc/postfix/mysql/relay_domains
~ Domains this mail server will relay (ie, send elsewhere)
user = myuser
password = mypassword
hosts = 127.0.0.1
dbname = mydatabase
query = SELECT domain FROM domains WHERE domain = '%s' and relay = 1 and enabled = 1
/etc/postfix/mysql/virtual_alias_domains
~ If you want to forward one domain to another domain, add the forwarding domain in the alias_domains table and add a route in the alias_maps table like @alias.com @goto.com.
user = myuser
password = mypassword
hosts = 127.0.0.1
dbname = mydatabase
query = SELECT domain FROM alias_domains WHERE domain = '%s' AND enabled = 1
/etc/postfix/mysql/virtual_alias_maps
~ User aliases eg mark@example.com goes to mark.long@example.com
~ Catchall: @example.com goes to mark.long@example.com
~ Nb. if you are using a domain that is not in the domains table, then you must add it to alias_domains table.
user = myuser
password = mypassword
hosts = 127.0.0.1
dbname = mydatabase
query = SELECT goto FROM alias_maps WHERE alias = '%s' AND enabled = 1
/etc/postfix/mysql/virtual_mailbox_domains
~ Virtual domains this mail server will handle
user = myuser
password = mypassword
hosts = 127.0.0.1
dbname = mydatabase
query = SELECT domain FROM domains WHERE domain = '%s' AND relay != 1 AND enabled = 1
/etc/postfix/mysql/virtual_mailbox_maps
~ Where to store a virtual mailbox's mail, prefixed by virtual_mailbox_base
user = myuser
password = mypassword
hosts = 127.0.0.1
dbname = mydatabase
query = SELECT maildir FROM mailboxes WHERE address = '%s' AND enabled = 1
/etc/postfix/mysql/transport_maps
~ What transport to use to send mail eg virtual or relay
~ Here you can put things like relay:[relay.mail-server.com] in the transport field
user = myuser
password = mypassword
hosts = 127.0.0.1
dbname = mydatabase
query = SELECT transport FROM domains WHERE domain = '%s'
Best Answer
Well I seem to have managed to figure this out with a bit of searching & testing. Here's what I had to do:
In /etc/postfix/main.cf:
In /etc/postfix/transport:
Relaying apparently also bypasses the alias_maps directive, so for aliases to continue working I had to comment out both alias_maps and alias_database, and replace them with virtual_alias_maps. The format of the virutal_alias_map is identical to alias_maps, so that was an easy change to make.
With these changes in place just restart postfix and also run "postmap /etc/postfix/transport" to build transport.db. Now everything addressed to @localhost or @localhost.localdomain is discarded while everything else is relayed through the specified host.