In recent past we experience the following behaviour of Mercury C (SMTP Relay Client): When sending big amounts of e-mails (in number and attachment) the transmitting speed to the ISP becomes more and more slowly over port 587 with SSL/STARTTLS. From time to time I become aware of the issue when someone of my users is expecting an urgent mail which doesn't come. Then I check Mercury for proper working and see that Mercury C is still sending one of a last e-mail since a long time (often about 1 hour). And as long as Mercury C is captured within the sending process, the other processes of the core module are stopped. (I report this Mercury issue already here ). In such case the SMTP queue runs over.
I can only improve the behaviour when switching Mercury C to another user's e-mail account, since all of our users have e-mail accounts with the ISP. Is it possible that ISPs reduce the SMTP mail speed more and more when receiving a lot of mails by one user to avoid spamming? Since all of our daily e-mails (hundret of mails) will be submitted to ISP over only one e-mail account, the provider could often think about spam.
Is anybody experiencing similar things? What are your SMTP upload speed over port 587? We've got a 50/10 Mbit (down/up) internet connection but the mail submission never reaches this speed. I observe the mercury C mail submission via the increasing of the SMTP log file (TCP-*.MC). BTW, our german ISP is "1&1". Maybe one of the german guys here are using the same provider.