Office 365, Email, Exchange, Office 365 Exchange Limits

Various limits on Exchange Online

Office 365 Email Limits
Office 365 Limits
Maximum number of messages per mailbox folder 1 million
Maximum number of messages per folder in the Recoverable Items folder 3 million
Storage quota for Recoverable Items folder 100GB
Maximum number of subfolders per mailbox 10,000
Maximum folder hierarchy depth 300
Message size limit 100 MB
Subject length limit 255 characters
File attachments per message limit 250
Messages received 3,600 per hour
Recipient rate limit 10,000 recipients per day and up to 1000 an hour for Employee
Recipient rate limit (Resource Accounts and 2FA enabled Users) 1,000 recipients per day and up to 500 an hour
Recipient rate limit (Non-2FA enabled Users) 100 recipients per day and up to 50 an hour
Recipient limit 500 per message
Message rate limit (SMTP client submission only) 30 messages per minute
Maximum number of distribution group members 50,000
Maximum message size for large distribution groups (>5,000 members) 2 MB
Exchange ActiveSync device limit 100
Exchange ActiveSync file attachment limit 25 MB max concurrent connections per mailbox limit 3
Junk Email folder retention period 30 days
Password length limit for Outlook on the Web 16 characters

Please also see:

Office 365, Exchange Online Protection, Recipient Limits