Question: I take videos in the MP4 format on my Android phone, and they play back just fine. But when I send a 20- to 60-second video to my friends’ phones (via Yahoo Mail or Facebook Messenger), the videos either won’t play or play for only five to 10 seconds. What’s wrong? — M.K., Minneapolis

Question: Either your recipients don’t have the right software to view your videos (not all MP4 files are the same), or you are trying to send too much data via Yahoo Mail and Facebook Messenger.

First, the software issue. When MP4 video files are created, they are compressed to minimize the amount of storage space they use. The files must be decompressed in order to be viewed. Android phones and iPhones support some of the different compression and decompression methods, but not all of them, which means some MP4 videos won’t play.

The solution for your recipients is to install a video app that handles most of these methods, such as the free and widely used VLC Media Player (see tinyurl.com/k3jyx4d), available for Android, iPhone, Windows PC and Mac. (For other video-playing programs, see tinyurl.com/yxmu8fuf for Android and tinyurl.com/y6bftugd for iPhone).

Then there’s the issue of file size. MP4 videos are big files that increase in size by about 20 to 30 megabytes per minute. However, Yahoo Mail and Facebook Messenger both limit the amount of data they transmit to 25 megabytes per message. As a result, your 60-second videos (20 to 30 megabytes) might be too big to fit into a message, and even your 20-second videos could exceed the limit if you send too many at once. If you exceed the limits, the video probably won’t be sent. If it is sent, the video may be cut off when the 25-megabyte limit is reached, which means the file might not work when your recipient tries to view it.

An alternative to sending long videos is to upload them to a cloud storage service such as Dropbox or Google Drive, then “share” them with your friends by sending them a Web link. (For details, see tinyurl.com/y43xf657 for Dropbox or tinyurl.com/yyyg3c4j for Google Drive.)

Question: Gmail used to do a terrific job of removing spam from my email inbox. But recently they don’t seem to be filtering out spam as well. What’s wrong and what can I do about it? — M.V., Eau Claire, Wis.

Answer: There could be a couple of reasons you have noticed more spam in Gmail this year.

One is that Gmail had a technical problem in July that slowed the receiving and sending of mail.

That in turn caused mail-screening problems that resulted in extra spam and promotional email being delivered to inboxes. Gmail said soon afterward that the problem had been fixed.

Another reason you might be getting more spam is a phenomenon known as “spam bombing.” This is an attempt to overload your inbox with messages in the hope that you’ll miss important alerts, such as one notifying you that a financial account has been compromised.

In addition, some spam senders might have acquired your email address for the first time.

Spammers expand their email lists by searching websites for addresses, trying commonly used account names, tricking people into giving out addresses and buying email lists.

About the only thing you can do is designate unwanted messages as spam and hope Gmail filters them out next time.