Wonder if any of you encounter a higher bounce (both hard and soft) this month, such as double (5-6%) than in the previous months (2-3%)?
A couple of the bounce details for records that were not bounced in the past:
550 Unable to add --- because host ---- is listed on bl.spamcop.net
550 JunkMail rejected - ---.mktomail.com [---- is in an 550 RBL: Blocked - see http://www.spamcop.net/bl.shtml?---
550 spamcop.mimecast.org Blocked - see http://www.spamcop.net/bl.shtml?---
550 [internal] [oob] The response text could not be identified.
Is this related to Marketo's servers, or to the sender servers?
not this month, but we experienced similar a couple of times in the past, even with the most engaged lists (and we really try hard not to spam people). It happens, and it is nothing particularly unusual. Also, your bounce rates are still not that hight that you'd need to particularly worry.
It looks your emails were blocked by spam filters. There can be many reasons why this happens, from your email appearing spammy (and trigger spam filters) to misbehaved neighbours (if you are on shared IP).
Additionally, search the community, there are many threads on this topic.
Did you (or an admin) happen to get a blacklist notification from Marketo? (Or are you on a dedicated IP?) Most of those error codes are because you are blacklisted, specifically with SpamCop; so you may have a spamtrap email address that entered your database recently that you emailed, causing you to get blacklisted (which ISPs use to bounce emails back for suspected SPAM).
Are you on a shared IP? If yes, then this can usually happen.
If you are sending an email using shared IP, there could be hard bounces caused by other customer who share the same range of IP for sending emails. Please refer the below article to know more about how shared IP range works and what are the other possible option you have.
Also, Spamcop listings are temporary and they clear within 24 hours. This means you should be able to resend your email to those addresses affected by this blacklist after the full 24 hour period.