Postfix: multiple relayhost for 2 internet lines

In the end I used policyd which does just this sort of thing. I configured it to limit a single IP address to no more than 10000 messages an hour and 1 GB. Below is a copy of the doc a wrote up for myself in case anyone finds it helpful.

Configuring postfix and policyd

After the packages have been installed make the following changes to their configurations. In /etc/postfix/main.cf add the following line

smtpd_client_restrictions = check_policy_service inet:127.0.0.1:10031

In /etc/postfix-policyd.conf

• change WHITELISTING=1 to WHITELISTING=0
• change GREYLISTING=1 to GREYLISTING=0
• change SENDERTHROTTLE=0 to SENDERTHROTTLE=1
• change QUOTA_EXCEEDED_TEMP_REJECT=1 to QUOTA_EXCEEDED_TEMP_REJECT=0
• change SENDER_QUOTA_REJECTION="Quota Exceeded." to SENDER_QUOTA_REJECTION="Quota Exceeded, 10,000 messages/hour max!"
• change SENDER_SIZE_REJECTION="Message size too big." to SENDER_SIZE_REJECTION="Message size too big. 10Mb per message or 1 Gb/hour max!"
• change SENDERMSGLIMIT=512 to SENDERMSGLIMIT=10000
• change SENDERRCPTLIMIT=3600 to SENDERRCPTLIMIT=10000
• change SENDERQUOTALIMIT=250000000 to SENDERQUOTALIMIT=1000000000
• change SENDERMSGSIZE=10240000 to SENDERMSGSIZE=50000000
• change SENDER_INACTIVE_EXPIRE=31d to SENDER_INACTIVE_EXPIRE=1h

is it possible to rent a server and use it as a Mail Server

Definitely, this is the solution my company implements. If you're not too worried about the time it takes to open an SMTP connection, you could use any free (gmail) account to send your mail too - you'll still need DNS config, and it's slow, but you'll get the mail sent as if it were despatched from the account itself.

I have recently answered some of these points. In summary, for config and prevention of "spam" tagging:

• You can check blacklists here
• Your server must have MX and reverse DNS records
• You must always have SPF DNS records (many servers reject mail without a valid SPF, GMail for example, here's an explanation and a wizard)
• For general config, your mailserver's HELO response must match your hostname
• Your mailserver is not an open relay - it must be a "closed" relay dedicated to your server - see how here
• Finally, ensure your DNS records' TTL is not too low - 86400 (24 hours) is recommended (some spammers set their TTL very low to regularly update forged DNS records)

Any suggestions for other Mail Server software instead of Postfix?

Not unless you use a remote SMTP server from your code, and this is slow. Qmail is the only real linux alternative: it's slow, cumbersome and irregularly updated. I wouldn't recommend it.

Another problem is Reverse DNS for this server. Should PTR entry be present if a server is used as a relayhost?

I believe DNS records (including reverse records) are always required: there are many different providers who check slightly varying parameters, so be sure to verify your configuration on every major provider (if possible!)