We recently migrated to an Office 365 solution with Microsoft hosted Exchange. We run quite a few linux servers for other processes of our company, and one of these servers sends mail on a very routine basis, sometimes generating as many as a few hundred
reports being emailed to various recipients within the organization inside of an hour's time. Our first couple weeks on Exchange, we had few issues, but this morning it seems our Exchange server stopped receiving mail from this server. Looking at the EXIM
log files on the sending server, I see countless errors like this:
SMTP error from remote mail server after end of data: host <our linux server>: 451 4.7.500 Server busy. Please try again later. (AS780081)
When telnetting to our Exchange server to attempt sending mail by hand, I receive the same error message. When sending mail from our linux server to outside servers, such as Yahoo, gmail, our own personal email accounts, we are experiencing no issues. After
googling for what seems like the whole day, I've only found 1 reference to this error code (AS780081) with the commenter suggesting this is the code sent when Microsoft thinks it sees suspicious activity.
http://community.spiceworks.com/topic/1190267-office-365-email-issue
After reading that, I've added the linux server as an allowed IP under Exchange's connection filter, added the sending addresses to the allow lists in the spam filter, and the sending subdomain to the allowed domains in the spam filter. Still, no mail is
getting through. Any ideas?