email not being delivered sporadically

email not being delivered sporadically

I often email SOs, POs, Invoices, etc to myself.  This used to work fine, I'd immediately get the message from Zoho Books. For the last several months or so the emails do not get delivered to me in a timely fashion.  Once in a while one will go through immediately, but more often than not I will get a large pile of undelivered mail all at once a week or more after having sent them.   I have thoroughly checked my email server and it is accepting mail from every other source I test when Zoho says it couldn't deliver the messages.    This seems like a bug in Zoho email delivery...?

-Tom