As a side note, new limits will be introduced in Exchange Online and therefore also in Office 365 from 01.06.2018. Specifically, these are new limits for authenticated SMTP connections (SMTP Authenticated submission):
- Sent mails are saved in the "Sent items" folder of the mailbox
- Only three simultaneous connections per mailbox are permitted
Depending on the configuration, you could run into problems here. It becomes problematic when devices such as scanners or multifunction devices use a service account to send emails. Software for sending e-mail newsletters can also be affected by the new limit.
In concrete terms, this means the following: Only 3 simultaneous authenticated SMTP connections are possible per mailbox. If multifunction devices, the monitoring software and the newsletter tool all use a service account with the name "noreply", for example, you will quickly reach this limit depending on the volume of mail. Although the connection can simply be repeated at a later time, multifunction devices in particular do not usually support this. Such devices usually only display an error and do not try to get rid of the mail again.
The following graphic shows a limit: 4 clients try to send their mails simultaneously with the user "noreply" and a password. Three of the connections are accepted, the fourth will receive the following error:
- 4.3.2 STOREDRV.ClientSubmit; sender thread limit exceeded.
As the mails now also appear in the "Sent items" folder in the "noreply" mailbox, it is a good idea to configure appropriate retention policies and automatically delete the mails after a certain period of time. Although the mailbox size should not be a problem with Exchange Online and Office 365, nobody normally looks at the sent mails of a service account. The emails therefore do not necessarily have to be saved forever.
Especially if many printers or newsletter programs are configured to the same service account, there is a risk that more than 3 simultaneous connections may be established. In this case, the error mentioned above would occur.
The only alternative is to use a separate mailbox / service account for each device or service. Of course, this also results in additional costs. Although not every printer has to use its own account, it may be possible to spread things out a little by, for example, all printers on one floor of an office building using their own service account.
Newsletters (bulk mail) should rather be processed via special services and not via Exchange Online or Office 365.