I have users that had been able to send to 150 recipients or so in the past with v15.x for years....for years - many sends. I know what you're thinking - "They should use an email list." That is what I thought and I sat down and examined their use case. They output dynamic lists of users from a database that fits the circumstance or topic of the email. So a static email list that can only be adjusted by adding/removing was not the solution for them. But, emailing was not an issue. It worked. They could send to approx. 150 recipients without issue with previous versions of SmarterMail.

Since switching to newest version of SmarterMail and using the MAPI protocol, sending this way for them now fails. They can only send to less than 40 recipients. I watched the spool and queue when troubleshooting and noticed that it seems SmarterMail sends in chunks of about 26 recipients and the remaining emails to be sent fail and fail with no error or warning message to the user or server admin.

Do I need to optimize my server so it can handle this many recipients? The stated limit by Microsoft for Outlook is 500. We are no where near that limit.

I have also noticed that when MAPI syncs an account for the first time, it syncs in smaller chunks than EAS or IMAP. Probably by design for stability and performance but it seems there is some kind of relationship to my users' problem.

I stuck with the 15.x version a long time because it worked. I'm regretting the upgrade to v17 with MAPI. My customers are very frustrated because I have assured them that the newer version with MAPI features would be better and is worth paying an additional fee.