Database Mail has a couple of log tables in the msdb database. These can become large over time. I've seen MSDB databases over 1 GB in size, where normal size is less than 50 MB (heavy usage of old SSIS deployment model excluded).
Unfortunately Maintenance Plans do not have built-in functionality for this, nor does Ola Hallengren's excellent maintenance solution ( http://ola.hallengren.com/ ). All you have to do is to schedule an agent job to be executed, say, every week, having one T-SQL jobstep containing:
DECLARE @DeleteOlder DATETIME
SET @DeleteOlder = DATEADD(MONTH, -1, CURRENT_TIMESTAMP)
EXECUTE msdb.dbo.sysmail_delete_mailitems_sp @sent_before = @DeleteOlder
EXECUTE msdb.dbo.sysmail_delete_log_sp @logged_before = @DeleteOlder
Above removes mail history older than one month. Adjust to your liking, using the values in the DATEADD function.
As always, remember to comment your job and to specify appropriate database for the T-SQL jobstep (for documentation purposes, msdb in this case).