Mailbox Storage Limit
UF mailboxes have a 100 GB storage limit. Unless you attempt to import many extremely large files, you should not experience any issues. If you reach 99 GB, you will receive a warning and be unable to send email. At 100 GB, you will be unable to send or receive email.
Message Size Limit
Individual email messages have a limit of 35 MB including attachments. This is for both incoming and outgoing messages. Email systems that you send to may also have message size limits so you may receive an error message from an external email system if you exceed their limits.