How does priority work in mx records and relays?

Question

SMTP

Our company has an Exchange 2003 mail server. Some users complain that some emails are not being received. Those. letters come, but some are "lost"
somewhere dig domain.com -t mx gives the following:
domain.com. 41970 IN MX 100 relay.provider.com.
domain.com. 41970 IN MX 50 relay2.provider.com.
domain.com. 41970 IN MX 10 mail.domain.com.
At relay2.provider.som port 25 is closed, on the other two addresses it is open.
I have read that the highest priority entries are only used if the lowest priority entry is not available.
I would like to know if the users' complaints are related to the fact that one record (namely relay2.provider.com.) is essentially not working, although logically, if it is not available, the record 100 relay.provider.som will be used.
The idea that my problem is related to MX records prompted the Microsoft Remote Connectivity Analyzer report, which checked two records at once and gave the answer "Connectivity test failed" precisely because it could not connect to relay2.provider.com.
And if it's not difficult for anyone to briefly describe how the relays work in the general case.

Answer 1

We don’t have Exchange, but with such a problem, I would look at the logs of my mailer for the subject: how they agree among themselves, DNSBL, Greylist.

Answer 2

The sending server will go to mail.domain.com, if it is unavailable or cannot accept mail for the user, then in most cases it will go to the second relay2.provider.com, if the sending failed, then it will go to the third one. From experience, the behavior of sending servers is very dependent on the software on the server and its settings. An attempt to resend can be made immediately, then if it did not work out, then after 15 minutes and hammer it for two days. As it should be - described in the RFC, in fact, anything can happen.
In your case, I would remove "domain.com. 41970 IN MX 50 relay2.provider.com", because Port 25 is closed and mail cannot be sent to it. As for the disappearance of letters, always ask users for the time, server ip address (if possible), sender address and recipient address, and look at the logs for a connection attempt. No connection attempts - no cartoons. :-)
About the loss of letters, I once wrote a post. Where do letters go?