Summary
Solano College mailboxes are subject to enforced hourly and daily sending limits that block a sender for 24 hours when exceeded. The limits reduce the impact of account compromise, phishing, and spam originating from College mailboxes, and they steer mass communications toward the correct tools.
Use this when: Planning a large email to students or employees, responding to a "message not delivered" or send-blocked notice, or advising a department that needs to reach a wide audience.
Audience: Faculty, staff, managers, and any District mailbox owner who sends to multiple recipients.
Authority: Acceptable Use of Technology Standards (AP 3910.7)
Applies to: All District Microsoft 365 mailboxes, including individual (faculty & staff), shared, and departmental mailboxes sending through Exchange Online.
Overview
To reduce phishing and spam sent from College mailboxes, effective fall 2026 Technology Services enforces sender limits on all Microsoft Exchange Online (i.e., Outlook) accounts. These limits cap how many recipients a single account can address per hour and per day, and they automatically restrict any account that exceeds them.
The limits apply to every District mailbox year round, except during the semester start window described below. Outside that window they are not adjusted for individual campaigns, and they cannot be waived at the mailbox level. Departments that need to reach large audiences should contact the Public Information Office (PIO) rather than sending directly from a mailbox.
Extra room at the start of a semester: Technology Services knows that the busiest outreach of the year happens right as classes begin. To support faculty and staff, sending limits are lifted for a two week window around the start of each full term fall and spring semester: the week before the first day of instruction and the first week of instruction.
Send your welcome messages, syllabus notes, class updates, and department announcements during that window without worrying about a send block. Outside the window, the standard hourly and daily limits apply again, so plan large audience messages accordingly or contact the Public Information Office.
Why this standard exists
Recent security incidents at the College have shown that a single compromised mailbox can be weaponized to send phishing messages to students, employees, and outside contacts within minutes. Without sending caps, a compromised faculty or staff account can:
- Blast phishing lures to the entire student body or employee directory before Technology Services can respond.
- Damage the District's sender reputation, causing legitimate District email to be marked as spam or rejected by outside providers.
- Expose students and employees to credential theft, financial fraud, and account takeover.
- Trigger downstream investigation, password resets, and Microsoft 365 remediation across many accounts.
Enforced limits contain the impact of an incident so a single compromise cannot become a district-wide event.
Sending Limitations
- External recipients per hour: 100. Any address that is not a Microsoft 365
@solano.edu mailbox counts as external, including @students.solano.edu Google accounts and personal email addresses.
- Internal recipients per hour: 250. Applies to messages sent only to
@solano.edu Microsoft 365 mailboxes.
- Maximum recipients per day: 500 across all destinations combined.
- Over-limit action: The sender is automatically restricted from sending any mail for 24 hours.
Recipient counts are cumulative across each message and each address on To, Cc, and Bcc lines. Distribution groups expand to their members for the purpose of these counts.
What faculty and staff need to do
Before sending any message that will reach a large group, apply the following steps.
- Estimate the total number of recipients, counting each address on To, Cc, and Bcc, and expanding any distribution groups.
- Classify each recipient as internal (
@solano.edu Microsoft 365 mailbox) or external (@students.solano.edu, personal addresses, or any non-District domain).
- Compare the totals against the hourly and daily limits above. If the message will exceed any limit, do not attempt to split it across multiple hours.
- Route mass communications through the PIO office and approved marketing tools instead of a personal or departmental mailbox.
Exceptions and special circumstances
Outside the semester start window described in the Overview, the limits are enforced district wide and cannot be waived on a per-message basis. Technology Services will continue to gather feedback from faculty and staff on how we can better support our College's email needs while balancing the security requirements of an evolving threat landscape. If a legitimate business need is not served by the current limits or by the tools available through the Public Information Office, tell us. Submit a Technology Services request or contact the Help Desk so we can review it as this standard develops.