Some points from our experience with this hosting hundreds of journals, some with many users.
Depending on the normal volume of messages sent in the SMTP used, a sudden increase in sending can cause some temporary blocks
Some users mark OJS messages as spam by mistake. In mass mailings this tends to occur more frequently, significantly increasing the possibility of other messages from OJS falling into the spam folder for other users.
If the OJS in question suffers from automated registration (bots), it is important to block the entry of new bots, if this still occurs, and clean the user base.